caranya dan jawabannya

Materi : SPLDV


2x + 9y = 21  |×2|
4x - 5y = 19   |×1|

Eliminasi
4x + 18y = 42
4x - 5y = 19
------------------- (-)
23y = 23
y = 1

subtitusi
2x + 9y = 21
2x + 9(1) = 21
2x = 21 - 9
2x = 12
x = 6

jadi
x/3 - 3y 
= 6/3 - 3(1)
= 2 - 3
= -1