I am not understanding your question. This is what the problem is (-7y^2-4y+6)=(7y^2-4y-4)-(2y^2-2y-6)Simplifying a sum or difference of three univariate polynomials
just collect like terms (same power)
(-7y^2-4y+6) = (7y^2-4y-4)-(2y^2-2y-6)
-7y^2-4y+6 = 7y^2-4y-4-2y^2+2y+6
-7y^2-4y+6 = 5y^2-2y+2
-12y^2-2y+4 = 0
Not sure what the equal sign is doing there. Simplifying an expression is not the same as solving an equation.