-x+2y
You have 3x+y and you need to make it 2x+3y
The difference between 3x and 2x is 1x(x)
And the difference between y and 3y is 2y
For the x you have to minus x from 3x to get 2x,(3x-x=2x)
For the y you have to add 2y to y to get 3y,(y+2y=3y)
Now add them
=3x+y+-x+2y=2x+3y
Therefore you add -x+2y
