Answer:
Yes, Sierra is correct
Step-by-step explanation:
Given

Required
Is she correct
The right parenthesis has 4 + 5.
This can be simplified as: 
So, we have:


The left parenthesis has y + 7.
This cannot be simplified because y is a variable and 7 is a number.
<em>Hence, Sierra is correct.</em>
Dat means 18+'a number'
aka
18+x