a) 8b + 5 = 133
The equation is going to be Cost of Books + Cost of Shipping = Total Cost.
In this case, the Total Cost is equal to $133, so we know that has to be the single number behind the equal sign. On the other side, you have to add the two variables going into the Total Cost: the Shipping Cost and the Book Cost. Given that we know that b is equal to the quantity of books being bought, we know that we would need to multiply that by the cost of each of those books; we are told that it is $8. Therefore, to represent the total cost of the books, we use 8b. We are also told that the Shipping Cost is $5, so that must be the other blank.
b) b = 16
Once we have the equation 8b + 5 = 133, we can solve for b. First, we can subtract 5 from both sides of the equation, which will leave us with 8b = 128. Then we can divide both sides of the equation by 8, giving us b = 16. This means that Ivan bought 16 books.