Distribute the 5 to the (2+y). Parenthisis always come first. after that combine the 5y and the -y. you should end up with 2-(+4y) +2. add the -2 and +2. you should be left with 4y
I assume you're looking for the number:
0.5n + 9 = 21
- 9
0.5n = 12
÷ 0.5
n = 24
So the beginning number is 24, hope this helps!