In general reducing A by B means we subtract B from A.
Question says Reduce 7y + 2y^2 -7 by 3-4y
It means we subtract 3 - 4y from 7y + 2y^2 -7
We use vertical format for subtracting
First we arrange the terms
7y + 2y^2 -7 can be written as 2y^2 + 7y -7
3 - 4y can be written as 0y^2-4y +3
Now arrange one by one in vertical format and subtract. While subtracting we change the sign of bottom expression so 0y^2 -4y + 3 becomes -0y^2 + 4y -3
2y^2 + 7y -7
-0y^2+4y -3
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2y^2 + 11y -10
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