You can put this solution on YOUR website! Not enough information. AC is definitely a diagonal, but points A and C can lie anywhere on the graph of y, the edges of ABCD do not necessarily have to be parallel with the axes, and knowing the length of the diagonal does not give any information about the rectangle's area (unlike a square).
Update: I have no idea how they got -1. -1 cannot be the only answer because if the entire rectangle was reflected about the x-axis, p could be equal to 1. Seems like any value of p works.