The given formula 
Solve for y means we have to make y alone
\\
Add -10 on both sides, we get

As there is multiplication sign between r and y so to solve for y, divide both side by r

Answer:
The answer is x = 7. Hope this helps.
Answer: 320ft^3
Step-by-step explanation:
Assuming the pyramid is just a normal rectangular pyramid and isn't made of stairs...
10x8x12/3 = 320 ft^3