Create a function that takes a number a and finds the missing exponent x so that a when raised to the power of x is equal to b.
solve_for_exp(4, 1024) ➞ 5
solve_for_exp(2, 1024) ➞ 10
solve_for_exp(9, 3486784401) ➞ 10
a is raised to the power of what in order to equal b?