I need help, could someone help me solve this? Also please explain so I can do other ones on my own. Thanks in advance!

Answer:
x = 9
Step-by-step explanation:
Solving for [tex]x[/tex] the equation
First with add 6 in each term in the equation in order to solve for x
6+3x+(-6) = 21+6
3x = 27
Then we divide both term for 3
3x/3=27/3
resulting
x = 27/3
Reducing
x = 9