Solve f/2 - 9 = 13 for f.

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