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

I'm having a mental block here, and algebra not really being my开发者_运维知识库 thing, can you tell me how to re-write the JavaScript code below to derive the variable, c, in terms of a and b?:

a = Math.pow(b, c);
c = ?

Thanks!

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

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

log_b a = c