$$2x^2 + xy- y^2, - x^2 + 2xy + 3y^2$$ and $$3x^2 - 10xy + 4y^2$$
The sum of $$2x^2 + xy - y^2, - x^2 + 2xy + 3y^2$$ and $$3x^2 - 10xy + 4y^2$$ is calculated as shown below
$$(2x2 + xy - y^2) + (- x^2 + 2xy + 3y^2) + (3x^2 - 10xy + 4y^2)$$
$$= 2x^2 - x^2 + 3x^2 + xy + 2xy - 10xy + 3y^2 + 4y^2 - y^2$$
We get,
$$= 4x^2 - 7xy + 6y^2$$
Hence, the sum of $$2x^2 + xy - y^2, - x^2 + 2xy + 3y^2$$ and $$3x^2 - 10xy + 4y^2$$ is $$4x^2 - 7xy + 6y^2$$