To solve for x, we need to isolate the variable on one side of the equation.

First, let's simplify the equation by combining like terms:

21 - 3x = 21 + 3x

Subtract 21 from both sides:

-3x = 3x

Now, add 3x to both sides:

-3x + 3x = 3x + 3x

0 = 6x

Now, divide both sides by 6 to solve for x:

0/6 = 6x/6

0 = x

Therefore, the solution to the equation is x = 0.