Answer:
Assuming "the speaker takes up 1/6 of the internal volume"
volume is 9³ in³
you want the remaining volume, which is 5/6 of the total
(5/6)9³ = 607.5 in³
double that if you want the remaining volume for BOTH speakers....
All you have to do is solve for x. See the following steps below.
Step 1. Subtract

from both sides

Step 2. Simplify

to

If you mean the equation 1/5 (5x +12) =18 then your answer would be x=15.6 or in the exact form of 78/5
Explanation : Start off with distributive property 1/5•5x and 1/5 •12, your equation should end up with 1x +2.4 =18 continue by using the 2.4 and doing the opposite, you will have 1x+2.4-2.4=18-2.4, after calculation you will have 1x=15.6 ,you can finish there or you can divide the 15.6 by one witch is still 15.6 , therefore getting your answer that is x=15.6
Look at the answers and compare the side lengths in the diagram to those. You don't need any math to see the logical answer here, the answer is most likely 14.3 units.