Question

решите систему 3у+х=12; у-4х+9=0

Answer 1

$left { {{3y+x=12} atop {y-4x+9=0}} right.\\left { {{x+3y=12}|*4 atop {-4x+y=-9}} right.\\+left { {{4x+12y=48} atop {-4x+y=-9}} right.\-------\13y=39\y=3\x=12-3y\x=12-3*3\x=3$

Ответ : (3 ; 3)