Task: Negative base numbers
Java: import java.util.List;
import java.util.Map;
import java.util.Objects;

public class NegativeBaseNumbers {
    private static final String DIGITS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

    private static String encodeNegBase(long n, int b) {
        if (b < -62 || b > -1) throw new IllegalArgumentException("Parameter b is out of bounds");
        if (n == 0) return "0";
        StringBuilder out = new StringBuilder();
        long nn = n;
        while (nn != 0) {
            int rem = (int) (nn % b);
            nn /= b;
            if (rem < 0) {
                nn++;
                rem -= b;
            }
            out.append(DIGITS.charAt(rem));
        }
        out.reverse();
        return out.toString();
    }

    private static long decodeNegBase(String ns, int b) {
        if (b < -62 || b > -1) throw new IllegalArgumentException("Parameter b is out of bounds");
        if (Objects.equals(ns, "0")) return 0;
        long total = 0;
        long bb = 1;
        for (int i = ns.length() - 1; i >= 0; i--) {
            char c = ns.charAt(i);
            total += DIGITS.indexOf(c) * bb;
            bb *= b;
        }
        return total;
    }

    public static void main(String[] args) {
        List<Map.Entry<Long, Integer>> nbl = List.of(
                Map.entry(10L, -2),
                Map.entry(146L, -3),
                Map.entry(15L, -10),
                Map.entry(-4393346L, -62)
        );
        for (Map.Entry<Long, Integer> p : nbl) {
            String ns = encodeNegBase(p.getKey(), p.getValue());
            System.out.printf("%12d encoded in base %-3d = %s\n", p.getKey(), p.getValue(), ns);
            long n = decodeNegBase(ns, p.getValue());
            System.out.printf("%12s decoded in base %-3d = %d\n\n", ns, p.getValue(), n);
        }
    }
}

VB: Const DIGITS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
Dim Str(63) As String
Private Function mod2(a As Long, b As Integer) As Long
    mod2 = a - (a \ b) * b
End Function
Private Function swap(a As String, b As String)
    Dim t As String
    t = a
    a = b
    b = t
End Function
Private Function EncodeNegativeBase(ByVal n As Long, base As Integer) As String
    Dim ptr, idx As Long
    Dim rem_ As Long
    Dim result As String
    If base > -1 Or base < -62 Then
        EncodeNegativeBase = result
    Else
        If n = 0 Then
            EncodeNegativeBase = "0"
        Else
            ptr = 0
            Do While n <> 0
                rem_ = mod2(n, base)
                n = n \ base
                If rem_ < 0 Then
                    n = n + 1
                    rem_ = rem_ - base
                End If
                result = result & Mid(DIGITS, rem_ + 1, 1)
            Loop
        End If
    End If
    EncodeNegativeBase = StrReverse(result)
End Function
Private Function DecodeNegativeBase(ns As String, base As Integer) As Long
    Dim total As Long, bb As Long
    Dim i As Integer, j As Integer
    If base < -62 Or base > -1 Then DecodeNegativeBase = 0
    If Mid(ns, 1, 1) = 0 Or (Mid(ns, 1, 1) = "0" And Mid(ns, 2, 1) = 0) Then DecodeNegativeBase = 0
    i = Len(ns)
    total = 0
    bb = 1
    Do While i >= 1
        j = InStr(1, DIGITS, Mid(ns, i, 1), vbTextCompare) - 1
        total = total + j * bb
        bb = bb * base
        i = i - 1
    Loop
    DecodeNegativeBase = total
End Function
Private Sub Driver(n As Long, b As Integer)
    Dim ns As String
    Dim p As Long
    ns = EncodeNegativeBase(n, b)
    Debug.Print CStr(n); " encoded in base "; b; " = "; ns
    p = DecodeNegativeBase(ns, b)
    Debug.Print ns; " decoded in base "; b; " = "; p
    Debug.Print
End Sub
Public Sub main()
    Driver 10, -2
    Driver 146, -3
    Driver 15, -10
    Driver 118492, -62
End Sub