Find the magnitude of the complex number $5-12i$.
I know that the magnitude of a complex number $a+bi$ is given by the formula $\sqrt{a^2+b^2}$. So, in this case, I just need to plug in $a=5$ and $b=-12$ and simplify. I get $\sqrt{5^2+(-12)^2}=\sqrt{25+144}=\sqrt{169}$. I recognize that 169 is a perfect square, so I can take the square root without using a calculator. The final answer is $\sqrt{169}=13$. 