find the slope, if it exists, for the line containing the pair of points

(-15.2,5.6) and (-13.2, 4.5)

slope m= ?

The long way of doing this is as follows:

y = mx + b
5.6 = -15.2m + b
4.5 = -13.2m + b
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Two equations, two unknowns. Subtract equation 2 from #1 and solve for m.
I get -0.55 but check my work.

The short way is
(y1-y2)/(x2-x2) = ??