Answer
$\theta=53.13°$
Work Step by Step
$\theta=cos^{-1}\frac{u\cdot v}{|u||v|}=cos^{-1}\frac{(-6i-3j)(-8i+4j)}{\sqrt {(-6i-3j)(-6i-3j)}\sqrt {(-8i+4j)(-8i+4j)}}=cos^{-1}\frac{48-12}{\sqrt {36+9}\sqrt {64+16}}=cos^{-1}(\frac{36}{\sqrt {3600}})=cos^{-1}0.6$
$\theta=53.13°$
