Answer
$94.4^\circ$.
Work Step by Step
From what we learned: $\cos\theta=\frac{U\cdot V}{|U||V|}=\frac{(-3)\cdot6+5\cdot3}{\sqrt{(-3)^2+5^2}\sqrt{6^2+3^2}}=\frac{-3}{\sqrt{34}\cdot\sqrt{45}}=\frac{-1}{\sqrt{170}}$
Thus $\theta=\cos^{-1}\frac{-1}{\sqrt{170}}\approx94.4^\circ$.
