Question

Solve r*(1-c*d)=r*(1-c*(d-p))-f for f

Answer 1

f=r*(1-c*d)-r*(1-c*(d-p))

Answer 2

try this
r-rcd=r*(1-cd-cp)-f
r-rcd=r-rcd-rcp-f
r-rcd will cancel both sides
0=-rcp-f
f=-rcp