Explications étape par étape :
a. f(x) = mx + 5
f(2) = 2m + 5
2m + 5 = -1
⇔ 2m = -6
⇔ m = -3
b. f(x) = -3x + 5
f(-2/3) = -3 * (-2/3) + 5
⇔ f(-2/3) = 7
c. -3x + 5 = 15
⇔ -3x = 10
⇔ x = -10/3
