My apologise also for not writing in latex but I'm new and still trying to get to grips with it.
Making f the subject of:
my first step was to square, so I had:
(F+p)/(f-p) = ( D/d)^2
then multiply by (f-p)
F+p = (f-p)(D/d)^2
now I get stuck because if I now want to remove f from one side I have to subtract f but that will leave me without any f's, please help!