To see why that is true, use Rule 4b to subtract the top row from the second row, getting . Now use Rule 1 to take a factor (b–a) out of the second row: . Next two steps are to use the same two rules to subtract the top row from the bottom row and then take a factor (c–a) out of the bottom row. You then get . Now subtract the second row from the bottom row and take out a factor (c-b), and you'll get the first part of the question. I'll leave you to do the rest.