Answer
b)
Work Step by Step
From what we learned: $\cos\theta=\frac{U\cdot V}{|U||V|}=\frac{3\cdot1+7\cdot(-4)}{\sqrt{3^2+7^2}\sqrt{1^2+(-4)^2}}=\frac{-25}{\sqrt{58}\cdot\sqrt{17}}=\frac{-25}{\sqrt{986}}$
Thus $\theta=\cos^{-1}\frac{-25}{\sqrt{986}}\approx142.8^\circ$, thus the answer is b)