Answer:
The unknown number is 2.
Explanation:
Let x represent the unknown number.
It is multiplied by 9 then added to 3. Hence, it can be represented by the expression:

The sum is equal to the product of 3 and 7, or 21. Therefore:

Solve for x. Subtract 3 from both sides:

And divide both sides by 2. Hence:

In conclusion, the unknown number is 2.