1. the slope formula is rise over run or y2-y1 over/divded by x2-x1 which in your case would be 372-248 over 6-4 so you would write m=y2-y1 over x2-x1 or m=372-248 over 6-4 (please be careful the numbers are small and goes at the bottom of the letter)
The term (2 + 8) is described as the total amount of pieces of fabric. It makes it easier to put this term this way so you wouldn't have to do (8 x 5) + (2 x 5) and do the distributive property.