First you have to find the slope with the equation y2 - y1 / x2 - x1 , then once you have found the slope (slope = -2) you simply plug it into the point slope formula. y-0 = -2(x-5) solving it algebraically you should arrive at Y=-2x+10. (the 5 and 0 were plugged in are two of the X and Y points of the line which is why we plugged them into the X and Y values of the equation)
For either one of the deals, just find what one card equals and times it by how many cards the other equation has.
e.g. 0.35 cents divide by 10 = 0.035. And now, you times it by 12 since the other deal is 12 cards for 40 cents.
0.035 x 12 = 0.48
So, 10 cards for 35 cents is not the better deal.
