Answer
$\frac{\pi}{4}$ rad = $45^\circ$
Work Step by Step
$\mathbf{u}=<3,1>$ and $\mathbf{v}=<2,-1>$
$\mathbf{u}\cdot\mathbf{v}=3\times2+1\times(-1)=6-1=5$
$\|\mathbf{u}\|=\sqrt{3^2+1^2}=\sqrt{10}$ and $\|\mathbf{v}\|=\sqrt{2^2+(-1)^2}=\sqrt{5}$
${\theta}=\cos^{-1}\frac{\mathbf{u}\cdot\mathbf{v}}{\|\mathbf{u}\|\|\mathbf{v}\|}=\cos^{-1}\frac{5}{\sqrt{10}\sqrt{5}}=\cos^{-1}\frac{1}{\sqrt{2}}=\frac{\pi}{4}$ rad = $45^\circ$
