x | = | 3 |

| |

21 | 9 |

Cross multiply:
x * 9 = 21 * 3
Simplifying
x * 9 = 21 * 3
Reorder the terms for easier multiplication:
9x = 21 * 3
Multiply 21 * 3
9x = 63
Solving
9x = 63
Solving for variable 'x'.
Move all terms containing x to the left, all other terms to the right.
Divide each side by '9'.
x = 7
Simplifying
x = 7