if 3x-7y=10 and xy=-1, find 9x^2+49y^2
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(a-b)^2=a^2 - 2*a*b + b^2
(3x-7y)^2=(10)^2
9x^2 - 2*3x*7y + 49y^2 = 100
9x^2 - 2*21*1 + 49y^2 = 100
9x^2 + 49y^2 = 100-42
=58.
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