5 → 13
3 · 5 - 2 = 15 - 2 = 13
7 → 19
3 · 7 - 2 = 21 - 2 = 19
12 → 34
3 · 12 - 2 = 36 - 2 = 34
Therefore we have the equations
3e - 2 = 10 |add 2 to both sides
3e = 12 |divide both sides by 3
e = 4
3 · 10 - 2 = f
f = 30 - 2
f = 28
Answer: f = 28, e = 4
