Please help! Everyone keeps getting this wrong

4. A thin, copper washer of inner radius R = 51.0 mm and width d = 25.5 mm carries a unformly distributed total charge Q = 7.00 nC. Determine the z-component of the electric field, E., due to the washer at a distance z = 11.0 em along the washer's sym- metry axis. Although you could use caleulus to solve this problem, it can also be solved with some clever superposition. The washer can be represented by the superposition of a POSITIVE disk with the same surface charge density (v; = rrl(:'fi'?m) as the washer and a radius equal to the OUTER radius of the washer, R +d, AND a NEGATIVE disk with the surface charge density (-r_.t = WFH_:J'?'T'I) and a radius equal to the INNER radius, R. If vou add together the positive field from the positive disk and the negative field from the negative disk, it should be equivalent to the electric field of the washer