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