tolong ya jangan asal =) pake cara

f(x)= ax + b
f(3)= 3a + b = 13 
f(5)= 5a + b = 25 

2a = 12
a= 6

3a + b = 13
3(6) + b = 13
18 + b = 13
b = -5

2a + 3b = 2(6) + 3(-5)
= 12 - 15
= -3 (c) 

maaf kalau salah