It is impossible to solve for 2 unknown variables in an equation. In order to solve this, you must think mentally.
What two numbers can you add to make 22?
A good example is 15 and 8.
Can 15 and 8 be divided into smaller numbers?
Yes.
15/3 = 5
8/2 = 4
^
Notice how I'm dividing by the coefficient? *3*c and *2*f
So C can be 5 and F can be 4.
3(5) + 2(4) = 22
15 + 8 = 22
22 = 22