F(x) = ax+b ; f(1) = -1, f(3) = 7

f(x) = ax + b

f(1) = -1
a + b = -1 ... (1)

f(3) = 7
3a + b = 7 ... (2)

Eliminasi persamaan 1 dan 2:
a + b = -1
3a + b = 7
__________ -
-2a = -8
a = 4

Substitusi a = 1 ke dalam pers. (1) atau (2)
a + b = -1
4 + b = -1
b = -1 - 4
b = -5

Jadi, f(x) = 4x - 5