Since we have two unknowns in both equations, the solution will still consists the known.
For the part A
P=4a,
Dividing both sides by 4,
P/4 =a
Therefore, a = P/4
Part B:
3c - 5f = 55
Add 5f on both sides of the equation,
3c-5f+5f=55+5f
3c=55+5f
Now subtract 55 on both sides
5f = 3c - 55
Divide both side of the equation by 5 to get f,
5f/5 = (3c-55)/5
f = (3/5)c - 11