Chapter 5 - Inner Product Spaces - 5.2 Inner Product Spaces - 5.2 Exercises - Page 246: 45

$\theta=66.586^o$.

Let $u=(-4,3),\quad v=(0,5), \quad =3u_1v_1+u_2v_2$. The angle $\theta$ between $u$ and $v$ is given by the formula $$\cos \theta=\frac{\langle u, v \rangle }{\|u\| \cdot\|v\|}=\frac{0+15}{\sqrt{ 48+9}\sqrt{ 25}}=\frac{3}{\sqrt{57}}.$$ That is, $\theta=66.586^o$.