Question

Find the slope of the line passing through the points (-9,-6) and (8,-6)

Answer 1

Y2-Y1/X2-X1
0/17=0
Answer is 0

Answer 2

Slope is also known as the gradient. 
You can calculate the gradient with the formula
(y2-y1 / x2-x1) 

So. (-6--6/8--9)
      = (-6+6/8+9) 
      = (0/17)
     = 0