What are the coordinates of the point that corresponds to −3π/4 on the unit circle?

−3π/4

radians=degree*(pi/180)
degree=radians*180/pi
degree=-(3π/4)*180/π=3*180/4=-135°

-135°------------Quadrant II  (90°,180°)

we will define the X and Y Coordinate points on the Unit Circle
X2 + Y2 = r2 (Pythagorean Theorem)
r = Radius of the Circle = Hypotenuse of the Triangle 
135°-90°=45°
For Θ = 45°, we have X = 1*cos45° = √2/2 and Y = 1*sin45° = √2/2

for belonging to 2 quadrant
the X and Y Coordinate points
(-√2/2,√2/2)