Answer
$\frac{2i}{\sqrt{5}}-\frac{j}{\sqrt{5}}.$
Work Step by Step
If $v=ai+bj$ then the unit vector $u$ in the direction of $v$ is $u=\frac{v}{||v||}$, where $||v||=\sqrt{a^2+b^2}.$ $||v||=\sqrt{2^2+(-1)^2}=\sqrt{5}.$
Hence $u=\frac{2i-j}{\sqrt{5}}=\frac{2i}{\sqrt{5}}-\frac{j}{\sqrt{5}}.$