tolong dibantu ya.........

f(x) = ax+b
f(3) = 3a + b = 15
f(5) = 5a + b = 20
Kurangi, didapat :
-2a = -5
a = 5/2

Dan,
3a + b = 15
3(5/2) + b = 15
15/2 + b = 15
b = 15 - 15/2
b = (30-15)/2 = 15/2

Maka,
f(x) = ax+b
f(x) = 5/2 x + 15/2

Dan,
f(-2) + f(4) = (5/2 (-2) + 15/2) + (5/2 (4) + 15/2)
f(-2) + f(4) = -5 + 15/2 + 10 + 15/2
f(-2) + f(4) = 5 + (15+15)/2
f(-2) + f(4) = 5 + 30/2 = 5+15
f(-2) + f(4) = 20

a) f(x) = ax+b
f(3) =  3a + b
15 = 3a + b

f(5) = 5a + b
20 = 5a + b
a= 2,5
b = 7,5

b)  f(x) = 2,5a + 7,5

c) f(4) = 17,5
f(2) = 12,5

c) f