When dealing with discounts, to find the amount that will be subtracted from the price, you will need to move the decimal two places (I think, don't quote me on that) to the right and then multiply the cost of the item with the discount. Seeing as the discount is 10% off, we have to multiply the cost of the item times the discount. (If the price of the rs550 is $100, we would times 100 by 0.10 to get $90.00 for the price of the item.)
Slope is the first y - the second y divided by the first x- the second x. Therefore we take -8-4= -12 and -7-0= -7. Using the formula, we get -12/-7. The negatives cancel out and we get 12/7 which is A.