Posted by Jessica on Sunday, October 22, 2006 at 12:49pm.
y=-3 , x=3
x+y=21(*)
3x-4y=0 (**)
(*)=>x=21-y
Replace to (**):=> 3(21-y)-4y=0
=>63-3y-4y=0
<=>7y=63=>y=63/7=9
y=9 replace (*)=>x=21-9=12
=>x=12
y=9
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