(7x2y2+12xy2-3x2y+11)+(-12xy2+20x2y+11xy-5)

Remove parentheses.

7x2y2+12xy2-3x2y+11-12xy2+20x2y+11xy-5

Subtract 12xy2 from 12xy2.

7x2y2-3x2y+11+0+20x2y+11xy-5

Add 7x2y2-3x2y+11 and 0.

7x2y2-3x2y+11+20x2y+11xy-5

7x2y2-3x2y+11+20x2y+11xy-5

Add -3x2y and 20x2y.

7x2y2+17x2y+11+11xy-5

Subtract 5 from 11.

7x2y2+17x2y+11xy+6

Add (7x^2y^2+12xy^2-3x^2y+11)+(-12xy^2+20x^2y+11xy-5)