`\text{a)}`
`M + (5x^2-2xy) = 6x^2 +9xy -y^2`
`-> M = 6x^2 +9xy - y^2 - 5x^2 +2xy`
`-> M= (6x^2 - 5x^2) +(9xy+2xy) -y^2`
`-> M = x^2 - y^2 +11xy`
$\\$
`\text{b)}`
`M - (6x^2-4xy) = 7x^2 - 8xy +y^2`
`-> M - 6x^2 + 4xy = 7x^2 - 8xy+y^2`
`-> M = 7x^2 -8xy +y^2 + 6x^2 - 4xy`
`-> M = (7x^2+6x^2) +(-8xy-4xy) +y^2`
`-> M =13x^2 -12xy+y^2`