Answer
$$\theta=\cos^{-1}\frac{-9}{\sqrt{14}\sqrt{21}}=2.123~rad$$
Work Step by Step
$$\cos\theta=\frac{v\cdot w}{\|v\|\|w\|}=\frac{-9}{\sqrt{1+9+4}\sqrt{4+1+16}}=\frac{-9}{\sqrt{14}\sqrt{21}}.$$
Hence
$$\theta=\cos^{-1}\frac{-9}{\sqrt{14}\sqrt{21}}=2.123~ rad.$$
