Répondre :
on cherche a
a = f(x1) - f(x2) / x1 - x2
a = f(5) - f(8) / 5 - 8
a = - 4/3 - (-9) / 5 - 8
a = - 97/15
on cherche b
f(5) = - 4/3
a X 5 + b = -4/3
-97/15 X 5 + b = -4/3
- 97/3 + b = -4/3
b = -4/3 + 97/3
b = 31
f(x) = -97/15 x + 31