Which of the following describes the correct process for solving the equation 3x+6=21 and arrives at the correct solution?
a. divide both sides by 6 and then at 3 the solution is x=5
b. 562 both sides of the equation and then divide by 2 the solution is x=9
c. subtract 6 from both sides of the equation and then divide by 3 the solution is x=8
d. subtract 6 from both sides of the equation and then divide by 3 the solution is x=5