We are looking for D in this case. We want to separate D from the rest of the equation and get it on one side by itself. We have:
3D+8=-17 ,
First we notice that the D is attached to the 3, in other words it is multiplying with 3. Since the 8 is by itself we can send it to the other side by subtracting both sides by it. We then have:
3D= -17-8 , This simplifies to 3D= -25.
When we have the 3 multiplying to the D we have to do the opposite to get it to the other side and that is divide. We divide both sides by 3 and get:
D= -25/3