Diketahui fungsi f(x) = ax +b. Jika f(-1)= -8 dan f(2) = 13, nilai a+3b adalah..... a. 1 b. 2 c. 3 d. 4

f(-1) = a.(-1) + b = -8
f(2) = a.2 + b = 13

-a + b = -8
2a + b = 13
--------------- -
-3a = -21
a = 7
sekarang cari b nya
-a + b = -8
-7 + b = -8
b = -1

a + 3b = 7 + 3.(-1)
= 7 - 3 = 4 ( D)