Opgelost...:-)

SOLVE([2·s + 2·s + 2·s = 30, h + h + 2·s = 20, 2·f + 2·f + h = 13], [f, h, s])
[f = 2  h = 5  s = 5]
f := 2
h := 5
s := 5
s + h*f
15

Opgelost...:-)