A sequence is usually a set or an array of numbers that has a strict way for moving from the nth term to the (n+1)th term.
If ``f(n) = f(n-1) + c`` where ``c`` is a constant value, then ``f`` is an arithmetic sequence.
An example would be (where the first term is 0 and the constant is 1) is [0, 1, 2, 3, 4, 5, ... and so on] )
Else if (pun) ``f(n) = f(n-1) * c`` where ``c`` is a constant value, then ``f`` is a geometric sequence.
Example where the first term is 2 and  the constant is 2 will be [2, 4, 8, 16, 32, 64, ... to infinity ... ]
There are some sequences that aren't arithmetic nor are they geometric.
Here is a link to feed your brain : Sequence !
You're going to write a function that's going to return the value in the nth index of an arithmetic sequence.(That is, adding a constant to move to the next element in the "set").
The function's name is `nthterm`/`Nthterm`, it takes three inputs `first`,`n`,`c` where:
  - ``first`` is the first value in the 0 INDEX.
  - ``n`` is the index of the value we want.
  - ``c`` is the constant added between the terms.
Remember that `first` is in the index ``0`` .. just saying ...
def nthterm(a, n, r):
    return a + n * r
def nthterm(a, n, r):
    return a + n * r