You can put this solution on YOUR website! First using the f= equation, having only two of the variables, to substitute into the first two equations may be easiest.
d+e+(2e-7)=10 and e=(2e-7)+d+2
d+3e-7=10 and e=2e+d-5
d+3e=17 and 0=e+d-5
d=17-3e.
Let's use this formula for d and substitute into the 0= equation.
0=e+(17-3e)-5
0=-2e+17-5
-2e=5-17
2e=17-5=15
Maybe now would be easiest to find f.
f=2(e)-7 and you just found e...
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continue...?