Hint: 15^2x = (15^2)^x = 225^x = 4. Do you know about logarithms?
With logarithms if a^x = y then log_a(y) = x where log_a is the logarithm to the base a. The common bases are the natural base e and base 10. To get any base you use the formula log_a(x) = log_b(x)/log_b(a) where log_b is the logarithm in base b and you use b = e (natural) or b = 10 where a is any base that is a whole number typically greater than 1 (but it can be in between 0 and 1 not including those two numbers).