Answer:
-x^2+2xy
Explanation:
x^2 + y^2 -2xy + 1 +something = y^2 +1
This will get rid of the x and x^2 terms
Subtract y^2 from each side
x^2 + y^2 -y^2 -2xy + 1 +something = y^2-y^2 +1
x^2 -2xy+1 +something = 1
Subtract 1 from each side
x^2 -2xy+1 -1+something = 1-1
x^2 -2xy+something = 0
Subtract x^2 from each side
x^2 -x^2 -2xy+something = 0-x^2
-2xy+something = -x^2
Add 2xy to each side
2xy -2xy+something = -x^2+2xy
something = -x^2+2xy
We need to add -x^2+2xy
Remember a binomial is 2 terms