Answer: f = 44
I'm assuming the "-9" part is not part of the denominator
(f/2) - 9 = 13
f/2 = 13+9 .... add 9 to both sides
f/2 = 22
f = 22*2 .... multiply both sides by 2
f = 44
As a check, replace f with 44 and simplify both sides
(f/2) - 9 = 13
(44/2) - 9 = 13
22 - 9 = 13
13 = 13 ... answer is confirmed
