Diketahui rumus fungsi f dengan rumus f (x) = ax + b. Jika f(6) = 23 dan f(5) = 18, nilai f (-4) adalah ...

f(x) = ax + b
f(6) = 6a + b = 23
maka b = 23 - 6a
f(5) = 5a + b = 18
substitusi b = 23 - 6a ke persamaan di atas :
5a + (23 - 6a) = 18
5a - 6a = 18 - 23
- a = -5
maka a = 5
b = 23 - 6a = 23 - 6×5 = 23 - 30 = -7
f(x) = 5x - 7
untuk f(-4) = 5×(-4) - 7 = -20 - 7 = -27
jadi
f(-4) = -27

f(x)=ax + b
f(6)= a6 + b =23...(1)
f(5)= a5 + b =18...(2)

Eliminasikan pers. 1 dan 2

a6 + b = 23
a5 + b = 18
----------------- -
a1 = 5
a= 5/1= 5

Substitusikan nilai a kepers. 2
a5 +b = 10
5 x 5 + b = 10
25 + b = 10
b =10 -25 =-15

f (-4)=5 x -4 + (-15)
= -20 + -15
= -35

Jadi f(-4)= -35