density should be 1000 kg/m3 for water. In math, they often ignore atmospheric pressure in the hydrostatic pressure equation P(z) = P0 + ρgz with z = 0 at the top of the dam.
The Force can be obtained from the equation dF = PdA = (ρgz)w(z)dz where we have chosen P0 to be 0 and w(z) is the width as you descend in z: w = 3620 - (3620-3100)(z/110) (a linear expression for w(z))
We can integrate in order to find the Force on the dam:
F = Integral from 0 to 110 of ((ρgz)(3620 - 4.7273z)dz)
I'll leave you to do the integral (Just power rule)