The function p (x) = 8x-10 shows the profit, in dollars, for selling x books.

p (10) is the profit, in dollars, from selling 10 books. Replace every copy of x with 10 and simplify

p (x) = 8x-10

p (10) = 8*10-10 ... every x is replaced with 10

p (10) = 80-10

p (10) = 70

So the profit is $70