Chapter 7 - Section 7.6 - Vectors: The Dot Product - 7.6 Problem Set - Page 411: 28

$91.2^\circ$.

From what we learned: $\cos\theta=\frac{U\cdot V}{|U||V|}=\frac{11\cdot(-4)+7\cdot6}{\sqrt{11^2+7^2}\sqrt{(-4)^2+6^2}}=\frac{-2}{\sqrt{170}\cdot\sqrt{52}}=\frac{-1}{\sqrt{2210}}$ Thus $\theta=\cos^{-1}\frac{-1}{\sqrt{2210}}\approx91.2^\circ$.