PQRS is a parallelogram taken in order, with P(-2, -1), Q(3, 0) and R(4, 5).
The diagonals PR and QS bisect each other. Find the coordinates of S.

Midpoint of $PR=(\frac{-2+4}{2},\frac{-1+5}{2})=$ (1, 2). This must also be the midpoint of $QS$, so $\frac{3+x}{2}=1$ gives $x=-1$ and $\frac{0+y}{2}=2$ gives $y=4$; hence $S$ = (-1, 4). (9, 6) pairs the wrong vertices as opposite, computing $Q+R-P$; (1, 2) reports the point where the diagonals CROSS instead of the vertex $S$; (-1, 6) slips when solving the $y$ equation.