Isolate the d. Note the equal sign. What you do to one side, you do to the other.
Subtract 3.2d from both sides
5.7d (-3.2d) = 3.2d (-3.2d) - 8
2.5d = -8
Isolate the d. Divide 0.5 from both sides
(2.5d)/2.5 = (-8)/2.5
d = -8/2.5
d = -3.2
-3.2 is your answer for d
hope this helps