Reorder the terms:
(9 + y)(-2 + y) = 4y
Multiply (9 + y) * (-2 + y)
(9(-2 + y) + y(-2 + y)) = 4y
((-2 * 9 + y * 9) + y(-2 + y)) = 4y
((-18 + 9y) + y(-2 + y)) = 4y
(-18 + 9y + (-2 * y + y * y)) = 4y
(-18 + 9y + (-2y + y2)) = 4y
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Solving
-18 + 7y + y2 = 4y
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