Solution: Use the Pythagorean theorem since $\overrightarrowOA$ and $\overrightarrowOB$ are perpendicular. Compute $\|\overrightarrowOC\|^2 = (2\|\overrightarrowOA\|)^2 + (\|\overrightarrowOB\|)^2 = 4(25) + 144 = 100 + 144 = 244$. Thus, $\|\overrightarrowOC\| = \sqrt244 = 2\sqrt61$. Final answer: $\boxed2\sqrt61$. - Portal da Acústica