There are two ways:

First way -- the graphical approach:

Plot the two points:

 

Draw the line between them

 

From the lower point, draw a line vertically upward 
until the top of the line is exactly even with the 
upper point, like the green line below:

 

Count the number of units the vertical green line is in 
length by comparing it to the units on the y-axis. We
see that it is 7 units in length.  So the numerator
of the slope is 7.

Next we draw a horizontal green line from the top of that
green vertical line over to the upper point, like this:

 

Count the number of units the horizontal green line is in 
length by comparing it to the units on the x-axis. We
see that it is 2 units in length.  Also the fact that the 
horizontal line was drawn to the left, the denominator
of the slope is taken to be negative.  So the denominator of
the slope is -2, so the slope is

  or



Second way -- the formula approach:

Use the formula 



where 

 and 



Edwin