Test 7 Section 7 - #12 (page 787)

If we're trying to identify the segment with a slope of -1, the first thing I want to do is eliminate the segments that have positive slopes.  We know any of these segments that are going up as we're going from left to right have a positive slope, so we're not even going to consider them.  That eliminates segments OC and OD.

OA, OB, and DC all have negative slopes.  If you've got a good eye for slopes,  you've got a good feeling about (E) right now, because OA looks way too steep, and OB looks way too gradual.  It's a good idea to confirm though, so let's put the points into the slope formula just to be sure.

DC goes between points (1, 3) and (3, 1).  So our slope is:

Yep, (E) is our answer.

Note that we can use this same formula to find the slopes of all the segments for practice, or just for fun (hah!):  Slope of OA = -3, slope of OB = -1/3, slope of OC = 1/3, slope of OD = 3.