What's the opposite of JavaScript's Math.pow?

Question

I\'m having a mental block here, and algebra not really being my thing, can you tell me how to re-write the JavaScript code below to derive the variable, c, in

Answer 1

Logarithms. You want the logarithm of a. B is the base, c is the exponent, so

log_b a = c

Answer 2

c = Math.log(a)/Math.log(b)