Answer
$81x^2+126xy+49y^2$
Work Step by Step
RECALL:
$(a+b)^2=a^2+2ab+b^2$
Use the formula above with $a=9x$ and $b=7y$ to obtain:
$=(9x)^2+2(9x)(7y) + (7y)^2
\\=81x^2+126xy+49y^2$
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