Answer :
Subtract 4d from each side. / / / 3d = 21. / / / Divide each side by 3. / / / d = 7 .
I'm not completely positive but I believe this is how you do it.
7d=4d+21
7d - 4d = 21
3d = 21
3d / 3 = 21 / 3
d = 7