Answer
$(-\frac{\sqrt 2}{2}, -\frac{\sqrt 2}{2})$
Work Step by Step
Because of the '-' in front of $\frac{3\pi}{4}$, you need to go counter-clockwise on the unit circle in the same distance, which gives you $\frac{5\pi}{4}$, which has the terminal point: $(-\frac{\sqrt 2}{2}, -\frac{\sqrt 2}{2})$