If 3x = 12, what is x?

To find the value of x in the equation 3x = 12, you need to isolate x. This can be achieved by dividing both sides of the equation by 3. When you do this, the equation becomes:

x = 12 / 3.

Calculating the right side gives:

x = 4.

Thus, the value of x is 4, which corresponds to the choice that indicates this value. This method of solving equations relies on maintaining equality by performing the same operation on both sides of the equation, leading you to the correct solution.