Pythagorean Theorem

a^2 + b^2 = c^2\\[10px]
\begin{aligned}
  c &= \sqrt{a^2 + b^2}\\[10px]
  &= \sqrt{3^2 + 4^2}\\[10px]
  &= \sqrt{25}\\[10px]
  &= \boxed{5}
\end{aligned}