Let's start with what we know. Since you're in middle school, you probably already have Order of Operations down. That's all this is. With the right order, you can solve this quick and easy :)
5/8 - 5/12 * (3-1/4) + 2/3
Start by finding the LCM of the different fractions. It's 24. You need to make the denominator of each fraction 24. Don't forget; whatever you do to one side of the fraction, you have to do to the other.
You should get 15/24 - 10/24 * (3 - 6/24) + 16/24
Now multiply everything by 24. You should get...
15 - 10(72-6)+16.
Now just solve!
15 - 10(66)+16.
15 - 660 +16.
-630 is your answer.
