let's eliminate the y's
1st times 7 --> 49x -63y = 91
2nd times 9 -> 81x + 63y = 819
add them:
130x = 910
x = 7
back into 2nd ...
9(7) + 7y = 91
7y = 28
y = 4
x=7 , y = 4
what is 7x-9y=13 and 9x+7y=91
elimination method
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