Answer
$$A=\frac{x^2+xy}{2}-y^2$$
Work Step by Step
We use the equation for the area of a triangle to find:
$$ A=\frac{1}{2}bh \\ A=\frac{1}{2}\left(x-y\right)\left(x+2y\right) \\ A=\frac{x^2+xy}{2}-y^2$$
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