Answer
$0$
Work Step by Step
By definition the dot product of 2 vectors $ai+bj$ and $ci+dj$ is $(ai+bj)(ci+dj)=ac+bd.$ If this product is $0$, then the vectors are orthogonal (perpendicular) and hence the enclosed angle is $90^\circ.$
Hence, here we have
$v\cdot w=(2i-j)(i+2j)=2\cdot1+(-1)\cdot2=0$
