The first thing you should do when solving equations is to check for like terms. If you do find any like terms, combine them.
6d - 3d + 2d - 6 = 4d
On the left-hand side, we can combine the d terms.
6d - 3d + 2d - 6 = 4d
5d - 6 = 4d
The next is to get the variable we're trying to solve for all alone. To do that, we'll have to add / subtract / multiply / divide on both sides of the equation.
5d - 6 = 4d
-6 = -d
Here I subtract 5d from each side of the equation which resulted the d variable almost being all alone. It's being multiplied by a -1, which is the reason why it has a negative sign. To get rid of the -1, divide both sides by -1.
-6 = -d
-6 / -1 = -d / -1
6 = d
So, d is equal to 6.