Question 628343
5f -|3f+4| = 20
5f - 20 = |3f + 4|

There are two cases: 3f + 4 > 0 and 3f + 4 < 0

Case 1: 3f + 4 > 0
|3f + 4| = 5f - 20
3f + 4 = 5f - 20
2f = 24
f = 24/2
f = 12

Case 2: 3f + 4 < 0
|3f + 4| = -(5f - 20)
3f + 4 = -5f + 20
8f = 16
f = 2

If you check f = 2, it is not the solution. Thus the only solution is f = 12.

Hope it helps:) 
John10