First, you have two expand the brackets on both sides.
Then you have to 'jump the fence' by putting the ones with letters on one side and the numbers on the other. When they jump the fence, they change from a negative to a positive and vice versa.
Then you simplify it, and you are left with -11y = -3.
So you have to divide it by -11 to make it just y and you do it to both sides, which gives you that answer.
1. expand everything
-9-20y+15 = -9y+9-6
2. get all the y's on 1 side and all the numbers on the other
3 = 11y
3. divide both sides by 11 to get 1 y
y = 3/11
4. if you are unsure about your answer check it by subbing y = 3/11 back into the original equation (you get both sides =6/11)