This is about variables. The problem is
d/2.5 - 3.4 = 4.6. What are the steps in finding out what the d is?
first, collect the stuff with d on one side, and the plain numbers on the other. Recall that a true equation remains true if you do the same thing to both sides.
So, start by adding 3.4 to both sides:
d/2.5 - 3.4+3.4 = 4.6+3.4
d/2.5 = 8.0
Now, multiply both sides by 2.5:
d/2.5*2.5 = 8.0*2.5
d = 20