[tex] \\ a)
\\ 2a^2b+3ab^2-5a^2b+6ab^2+9a^2b-4ab^2 =>
\\ 2a^2b-5a^2b+9a^2b+3ab^2+6ab^2-4ab^2 =>
\\ 6a^2b+3ab^2+6ab^2-4ab^2 =>
\\ \boxed{6a^2b+5ab^2} [/tex]
[tex] \\ b)
\\ \left(2x^2+4xy\right)+\left(y^2-2xy\right)-\left(x^2+4y^2-5xy\right)+\left(x^2-y^2\right)=>
\\ 2x^2+4xy+y^2-2xy-\left(x^2+4y^2-5xy\right)+x^2-y^2 =>
\\ 2x^2+4xy+y^2-2xy-x^2-4y^2+5xy+x^2-y^2 =>
\\ \boxed{2x^2+7xy-4y^2} [/tex]
