3/7d=4/7. What does d equal? Can someone help me please

The way you typed it ....

3/7d=4/7
multiply both sides by 7
3d = 4
d = 4/3

if you meant: 3/(7d) = 4/7
cross-multiply
28d = 21
d = 21/28 = 3/4

take your pick