The answer would be 21 because its the same length as the other side.
Answer:
This could be wrong, but i still wanted to help you.
Step-by-step explanation:
9-3x=y
+9 +9
3x=y
÷3 ÷3
x=y/3
Im sorry if this is wrong
<h2>
Hello!</h2>
The answer is:

<h2>
Why?</h2>
To solve the problem we need to add/subtract like terms. We need to remember that like terms are the terms that share the same variable and the same exponent.
For example, we have:

We have that we were able to add just the terms that were sharing the same variable and exponenr (x for this case).
So, we are given the expression:

Hence, the answer is:

Have a nice day!
He should take breaks every 15 minutes and he should also eat a healthy snack and drink lots of water