8xy-3y^2+5x^2-7xy+12x^2-2y^2-3xy+8y ^2

 8xy-7xy-3xy-3y²-2y²+8y²+5x²+12x²
then simplify by combining like terms -2xy+3y²+17x²

Can combine like terms
remember yo an move terms around
like terms examples
x and 2x are like
3xy^2 and 4xy^2 are like
x and 2x^2 are not
3x^2y and 3xy are not

group like terms together

I like doing it in power order, highest to lowest

5x²+12x²-3y²-2y²+8y²+8xy-7xy-3xy=
(5x²+12x²)+(-3y²-2y²+8y²)+(8xy-7xy-3xy)=
(17x²)+(-3y²)+(-2xy)=
17x²-3y²-2xy