Since x is already "alone", you can plug 3y-11 into the x-value for the first equation to get 3(3y-11)+5y=23. Now distribute 3 to the numbers in the parentheses. In the end, the solution to this is (1,4).
15$ I believe, as we can disregard the two original books (they have been payed for) leaving her to buy 1 book and having seven dollars, however if she needs $8 more you simply need to add 8+7.
