x
 = 
12
1
3

Cross multiply:
x * 3 = 1 * 12

Simplifying
x * 3 = 1 * 12

Reorder the terms for easier multiplication:
3x = 1 * 12

Multiply 1 * 12
3x = 12

Solving
3x = 12

Solving for variable 'x'.

Move all terms containing x to the left, all other terms to the right.

Divide each side by '3'.
x = 4

Simplifying
x = 4