on considère la fonction f: x → -x +8

pour x = -5 on a f(-5) = -(-5)+8
f(-5) = 5 + 8 donc f(-5) = 13
pour x = 0
f(0) = 0 + 8
f(0) = 8
pour x = 4
f(4) = -4 + 8
f(4) = 4

-x + 8 = 0
donc 8 = x
-x + 8 = -7
-x = -7 - 8
-x = -15
x = 15