Answer
$\theta=0$
Work Step by Step
$\theta=cos^{-1}\frac{u\cdot v}{|u||v|}=cos^{-1}\frac{(2i-j)(6i-3j)}{\sqrt {(2i-j)(2i-j)}\sqrt {(6i-3j)(6i-3j)}}=cos^{-1}\frac{12+3}{\sqrt {4+1}\sqrt {36+9}}=cos^{-1}(\frac{15}{\sqrt {225}})=cos^{-1}1$
$\theta=0$
