We have the equation:
8c + 7c + 6 = 66
We add the c terms, and add -6 to each side of the equation (to cancel the 6 on the left-hand side):
15c + 6 - 6 = 66 - 6
15c = 60
Now, we divide each side of the equation by 15:
15c/15 = 60/15
c = 4
So the value of the variable c is 4.