If $10^x - 10 = 9990,$ what is $x$ equal to?
I notice that the equation has $10^x$ and $10,$ which are both powers of $10.$ Maybe I can rewrite $9990$ as a power of $10$ too, or something close to it. I see that $9990 = 10^4 - 10,$ so I can substitute that into the equation and get $10^x - 10 = 10^4 - 10.$ Now I can add $10$ to both sides and get $10^x = 10^4.$ This means that $x$ must be equal to $4,$ since the only way for two powers of $10$ to be equal is if their exponents are equal. I can check my answer by plugging it back into the original equation: $10^4 - 10 = 10000 - 10 = 9990,$ which is correct. 