Answer
$\frac{3}{5}i-\frac{4}{5}j.$
Work Step by Step
If $v=ai+bj$ then the unit vector $u$ in the direction of $v$ is $u=\frac{v}{||v||}$, where $||v||=\sqrt{a^2+b^2}.$ $||v||=\sqrt{3^2+(-4)^2}=\sqrt{25}=5.$
Hence $u=\frac{3i-4j}{5}=\frac{3}{5}i-\frac{4}{5}j.$