Find $|{-324} + 243i|$.
I recall that the modulus of a complex number $a + bi$ is given by the formula $\sqrt{a^2 + b^2}$. So, to find $|{-324} + 243i|$, I need to plug in $a = -324$ and $b = 243$ into the formula and simplify. I get $\sqrt{(-324)^2 + (243)^2} = \sqrt{104976 + 59049} = \sqrt{164025}$. I notice that 164025 is a perfect square, since it is equal to $405^2$. Therefore, I can simplify the square root and get the final answer. 