You should have written the "problem" with parentheses as
3/(2y) -3/(2y+4) . The way you wrote it, it is not clear what the denomonators are.

You still have not asked a question.

You cannot solve for y since it is not an equation. If you are supposed to rewite it with a common deominator, that would be
3(2y+4)-3(2y)/[2y(2y+4)]
= 12/[2y(2y+4)] = 6/[y(2y+4)]