Question 970802
slope = changes in y/changes in x

may go in either direction, but MUST be consistent
left to right   9-1/7-1
=8/6 or 4/3

Do it the other way
1-9/1-7   -8/-6  =4/3

Be careful when the second number is negative

1,-1)  (3,4)

y=4 - (-1)/3-1         4-(-1)=5

5/2