Explanation:
the slope is always the ratio
y coordinate difference / x coordinate difference
when going from one point on the line to another.
in our case, when going from (4, 8) to (8, 6) :
x changes by +4 (from 4 to 8).
y changes by -2 (from 8 to 6).
the slope is then -2/+4 = -1/2
by the way, we could have gone the other way around and still would have gotten the same result, of course.
from (8, 6) to (4, 8)
x changes by -4 (from 8 to 4).
y changes by +2 (from 6 to 8).
the slope is +2/-4 = -1/2