So basically we're just looking for expressions equivalent to d - 12.50 = 17.
Let's look at each choice.
a.) d = 17 - 12.50
Try to make this look identical to d - 12.50 = 17
We can subtract 17 on both sides,
d = 17 - 12.50
- 17 - 17
We get:
d - 17 = -12.50
Subtract d on both sides
- 17 = -d - 12.50
or
-d - 12.50 = - 17
Divide -1 on both sides to all terms:
d + 12.50 = 17
Comparing this to d - 12.50 = 17, they do not look identical.
On to b): d = 17 + 12.50
Subtract 12.50 on both sides,
d - 12.50 = 17
This looks exactly like d - 12.50 = 17 so b) is the correct answer! :D