You required to find the value of d.
Always remember the concept that if you transpose "+" sign to the other side, it becomes "-" sign, and vice versa. Make sure that you arrange the like terms and constants. Solving the equation,
6d + 1 = 15 - d
6d + d = 15 - 1
7d = 14
d = 14/7 = 2
When transposing a multiplication, you divide it to the other side.
Verifying the solution,
6(2) + 1 = 15 - 2
12 + 1 = 13
13 = 13
