Okay, lets see...:)
I = r^2 dm --according to definition:)
rho = m/ab
dm = rho * da*db (since x and y are really a and b in this case--right?)
dI = r^2 * rho* da*db
dI = (r^2 * m * da* db )/ab
i will then have to do a double integration right? over limits 0 to a/2 and 0 to b/2 respectively?.. hmm..