Verified answer

Here's how you do it:

make a circle, then find where 7pi/4 is in the circle. as in, one pi is half the circle going from middle right to middle left. that's one pi. now divide the second half (bottom half) into 4 pieces and find the location of the end of the third piece. The Y in the circle is the location of your y in the cartesian's (x,y) coordinate.