fungsi f(x) = ax+b , jika f(2) = -2 dan f(-3) = 13. maka tentukan nilai f(4)...? mohon bantu teman...

f(x) = ax + b

f(2) =   2a + b = -2
f(-3) = -3a + b = 13
          ------------------- (-)
                5a = -15
                  a = -3

2a + b = -2
2(-3) + b = -2
b = -2 + 6
b = 4

rumus fungsi f 
f(x) = -3x + 4

f(4) = -3(4) + 4
      = -12 + 4
       = -8

f(2) = -2
2a+b = -2

f(-3) = 13
-3a+b=13

2a+b=-2
-3a+b=13
________ -
5a = -15
a = -3

2a+b=-2
-6+b=-2
b=4

f(4) =-3x+4
f(4) = -3(4) + 4
f(4) = -12 + 4
f(4) = -8