a.
\(M+\left(5x^2-2xy\right)=6x^2+9xy-y^2\)
\(\Rightarrow M=6x^2+9xy-y^2-\left(5x^2-2xy\right)\)
\(\Rightarrow M=x^2+11xy-y^2\)
b.
\(\left(3xy-4y^2\right)-N=x^2-7xy+8y^2\)
\(\Rightarrow N=\left(3xy-4y^2\right)-\left(x^2-7xy+8y^2\right)\)
\(\Rightarrow N=-x^2+10xy-12y^2\)
