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Hello from MrBillDoesMath! Answer: See answers below in Discussion section.Discussion: #1  (7x + 4)  - (2x + 9) =(7x - 2x) + (4 - 9 ) =5x -5#10(5x^4 - 2x^2) - (3x - 2x^2 - 4x^3 + 6x^4) =           => combine like termsx^4(5 - 6) + x^3(4) + x^2(-2+2) + x(-3) =                 => as =2 +2 = 0 =-x^4 + 4x^3 + x^2(0) + (-3)x == -x^4 + 4x^3 - 3x#11(3x^2 + 7xy  - 2y^2) - (x^2 - 6xy + 2y^2)  =x^2(3-1) + xy(7+6) + y^2(-2-2) =2x^2 + 13xy -4 y^2#12(-x^2 - 9xy+5y^2) - (4x^2 -2xy -y^2)x^2 ( -1 -4) + xy(-9 +2) + y^2(5+1) =-5x^2 -7xy + 6y^2 Thank you, MrB