Dm de maths, niveau 2nde, urgent svp!!!

bonjour,

1 ) fonction affine = ax+b
f(1,5) = -2  →1,5a+b = -2
f(3) = 1  →  3a+b = 1

1,5a+b = -2
3a+b = 1

-1(1,5a+b)  =-2
3a+b = 1

-1,5a-b = 2
3a+b = 1

1,5a = 3
a = 2

3a+b = 1
6+b = 1
b = 1-6
b = -5

f(x) = 2x-5

2) à toi, c'est la meme chose