Answer: y
Explanation:
For this, you would just add everything and then tag a y on the end.
3y-10y=-7y
-7y+y= -6y (y is the same thing as 1y)
-6y+7y= 1y which is just y
It is important to understand that you can only do this with like terms. So for example if you were asked to simplify:
5x+2x+3y+5y
the answer would be 7x+8y (you cannot combine x and y)
Hope this helps :)