You don't need to use the dot product here, in fact, all we need to do is construct the diagram.

This is what our diagram should look like: Parallelogram - Wikipedia, the free encyclopedia

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 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

The same procedure but for the x-cordinate between R and Q yields . Thus, the x co-cordinate for S is

S is at (8, 0)