a)
`A+B = (3xyz - 5xy + 4x^2) + (2x^2 + xyz + 5xy)`
` = 3xyz - 5xy + 4x^2 + 2x^2 + xyz + 5xy`
` = (3xyz + xyz) - (5xy-5xy) + (4x^2 + 2x^2)`
` = 4xyz + 6x^2`
b)
`A-B = (3xyz - 5xy + 4x^2) - (2x^2 + xyz + 5xy)`
` = 3xyz - 5xy + 4x^2 - 2x^2 - xyz - 5xy`
` = (3xyz - xyz) - (5xy+5xy) + (4x^2 - 2x^2)`
` = 2xyz -10xy + 2x^2`