The slope is the difference of y-coordinates divided by the difference of x-coordinates. Aritmetic is all that's needed.
20. (-4-3)/(7-(-2)) = -7/9
21. (1-(-1))/(4-(-6)) = 2/10 = 1/5
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The attached graphs show the lines in point-slope form, so the slope values computed above are verified to make the line go through the given points.