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First get the variable, d, on one side of equation like this:
14 -d + d = -7 + 6d + d (since d is negative on left side you have to add +d to take it away, and you add it to the other side)
Now you have 14 = -7 + 6d + d add the d's to get 14 = -7 + 7d
Now you have to get the d term by itself, like above you simply get rid of the negative 7 by moving it to the other side with the 14.
14 = -7 + 7d
14 + 7 = -7 + 7 + 7d (again, since it is a negative 7, you have to add +7 to it in order to take it away, but also add it to other side to the 14
Now you have 14 + 7 = -7 + 7 + 7d Combine the numbers by itself to get
21 = 7d
Now solve for d by dividing both sides by 7
d = 3