You can put this solution on YOUR website! A = (3(F-f))/L
Let's solve for f:
A = (3(F-f))/L is equivalent to (3(F-f))/L = A, so I'm going to write it this way:
(3(F-f))/L = A
Multiply both sides by L/3:
F-f = (AL)/3
Subtract F from both sides:
-f = (A L)/3-F
Finally, multiply both sides by -1:
f = F-(AL)/3