I got a different answer, maybe I made a mistake in the derivation but you can check to see if it's right though.
edit:
Oh I see my mistake now, when I absorbed (W/L)4c into the square root I forgot to square it. and (W/L)4c=P/M * (W/L)2E which would make
Rds=[2uCox(P/M)^2(W/L)2E*ID]^-1/2 which would equate to gm2E^-1/2 * M/P