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 add 3. the solution is x = 5.
b. add 6 to both sides of the equation and then divide by 3. 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.