find 4 + 7 + 10 + 13 + 16 + . . . up to 20 terms
a = 4 d = 7 – 4 = 3 sum of first 20 terms , s 20 answer is a
add(add(multiply(20, 16), multiply(13, 10)), multiply(const_10, subtract(multiply(7, 4), subtract(10, const_2))))