Hi student, let me help you out! :)
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We are asked to find the slope of the line that passes through (-6, 8) and (-9, 7).
(formula for Slope)

y2 & y1 = the y-coordinates of the two points
x2 & x1 = the x-coordinates of the two points
Remember this...

So we substitute the values and obtain:

Simplifying:

Simplifying more:

Divide the top and bottom by -1:

There's the slope.
Hope it helps you out! :D
Ask in comments if any queries arise.
~Just a smiley person helping fellow students