Answer: x=4, y=-1
Explanation:
Multiply the second equation by 3:
18x+9y=63
Then summarize both equations 18x+9y+3x-9y=21+63
21x=84 => x=4
use x=4 in any of initial equations ,for instance in first one:
3*4-9*y=21
12-9*y=21
9y=12-21
9y=-9
y=-1
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