So graph the points you have and label the remaining point in the vacent corner S and draw lines to it.
The height of the known side (point Q to point P) is \(\displaystyle 4-1 = 3 \) and the right side of the diagram has point R at height 3. We know that, by the shape of the parallelogram S needs to be bellow R, so its height or y co-ordinate is \(\displaystyle 3-3=0 \)
The same procedure but for the x-cordinate between R and Q yields \(\displaystyle -6-4 = -10 \). Thus, the x co-cordinate for S is \(\displaystyle -2 - - 10 = 8 \)