A square plate with edge a cm and areal density kr g/cm^2, where r is the distance (in centimetres) from P to one of the diagnals of the square.
Find the mass.

I know my diagram is wrong, and I know what the correct one looks like. However, I do not see how my diagram does not work... The two diagnals of the square meet at the centre which in my diagram is the origin. Hence, and radius extends from all the diagnals to position P.
Any ideas?