fungsi f (×) a×+b jika f (2) = 3 dan f (4) = 9 maka f (8) adalah ...

f(x) = ax + b

eliminasi b
f(4) -> 4a + b = 9
f(2) -> 2a + b = 3
------------------------ -
2a = 6
a = 6/2 = 3

subtitusi nilai a = 3 ke salah satu persamaan
2a + b = 3
2.3 + b= 3
6 + b = 3
b = 3 - 6 = -3

f(8) = 8a + b
f(8) = 8.3 + (-3)
f(8) = 24 - 3
f(8) = 21

semoga membantu ^-^