Answer:
4x^2 +9xy +6y^2
Step-by-step explanation:
Combine like terms. I find it useful to factor out the variable part of the expression so I can see what coefficients are being combined.
7x^2+5xy-3y^2 - (2x^2+3xy-5y^2 + x^2-7xy-4y^2)
= (7 -2 -1)x^2 +(5 -3+7)xy +(-3 +5 +4)y^2
= 4x^2 +9xy +6y^2
