Divide both sides by 3, since 15 can be divided by 3 to get a whole number:
[3(2Y-9)]/3 = 15/3
The 3 cancels on the left side and you get 5 on the right side:
2Y-9 = 5
Now add 9 to both sides to leave Y by itself:
2Y-9+9 = 5+9
2Y = 14
Now divide by sides by 2 to solve for Y:
2Y/2 = 14/2
Thus, Y = 7