Slope is y1-y2 over m1-m2 (rise over run). The first ordered pair is -2 (m1) and 11 (y1). We then subtract the second ordered pair (4 (m2) and -4 (y2)) from the first.
11 - (-4) = 11 + 4 = 15
-2 - 4 = -6
Remember, slope is rise over run (y over x), so the slope is 15/-6. Now, we must simplify. 15/-6 = 5/-2
Dean went wrong because he thought that slope was run over rise (x over y). If he had switched the two numbers, his answer would have been correct.
