The slope of every horizontal line is 0 because that is based on the formula. If it's horizontal on the x axis for example, the point slope will be 0,0, also the horizontal line is the numerator, i think. And the undefined slope is vertical because it's the denominator.
(1) let there be x book and let the total cost be y dollars.
y=19.25x+40
y=16x+69.25
(2) 19.25x+40=16x+69.25
3.25x=29.25
x=9
9 silver book, 9 gold book.
(3) y=19.25x+40=19.25*9+40=213.25 dollars
both companies' total cost are 213.25 dollars.
You have to find a common denominator first.
