Short answer:
TotalCost = 12.96
Long answer:
It gives you the equation of
total cost = C + 0.08C
You just need to plug-in the $12 to the equation.
In other words, the equation is:
TotalCost = 12 + 0.08*12
TotalCost = 12 + 0.96
TotalCost = 12.96
Answer: Josh sold 11 book shows and Jessica sold 198 books.
Look at the attachment for the work.
The answer is

If we put 36x^2, then the answer we would get is

Hope this helps :)