f= 0x+b,jika f(2)=12 dan f(5)=3 maka nilai f(-3)=...

f(x) : ax+b
f(2) : a(2)+b = 12
...... : 2a+b = 12
f(5) : a(5)+b = 3
...... : 5a+b = 3

2a+b = 12
5a+b = 3
------------- --
-3a = 9
.. a = -3
2(-3)+b = 12
-6+b = 12
..... b = 18

f(x) : -3x+18
f(-3) : -3(-3)+18 = 9+18 = 27