Making f the subject of:

Sqrt(f+p)/sqrt(f−p)=D/d

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!

Thanks

kris