Unless you know what the value of a and b is, there is no way to solve for simplify the problem. You have already done the problem with the answer: 3a-1=b
You can not simplify this anymore without the values of a and/or b.
The correct answer for the original price of the novel is $20. Since 30% of $20 is $6. So if you'll deduct the discount, the price will go down to $14.