Are you sure it's you need to find the size of, not ?

If so, we haven't got enough information about where A is, unless the square is positioned so the sides are horizontal and vertical.

If this is the case, then C is at point (2, 9), which means A is at (8, 3) (6 over from C and 6 down - you know it's 6 because the square is area 36.

So you need to find the area of the triangle (2,5), (6,9), (8,3).

Can you take it from there?