Remark
Let the cost of each book = x
9*x + 18 = 315 Subtract 18 from each side
9x + 18 - 18 = 315 - 18
9x = 297 Divide both sides by 9
9x/9 = 297 / 9
x = 33
Conclusion
Each book cost 33 dollars.
A small point.
You really don't have to offer this many points. 15 or even 10 will get someone's attention.