A box in an equation, like $8 \times \square = 48$, stands for a number we have not found yet. Your fact families find it.
Why it works
The equals sign means "is the same amount as." It does not mean "the answer comes next." So $56 = \square \times 7$ is a fine equation: 56 is the same as some number times 7. To find the box, ask what number makes both sides equal, and use a fact from the family.
Key idea
To find a missing number, turn the equation into a question. $8 \times \square = 48$ asks "8 times what is 48?" $35 \div \square = 5$ asks "35 shared into how many groups gives 5?"
Worked example: A missing factor
$8 \times \square = 48$
- 8 times what makes 48?
- $8 \times 6 = 48$.
Answer: $6$
Worked example: A missing divisor
$35 \div \square = 5$
- 35 divided by what gives 5? Same as: $5 \times \square = 35$.
- $5 \times 7 = 35$, so $35 \div 7 = 5$.
Answer: $7$
Worked example: The equals sign first
$56 = \square \times 7$
- Both sides are the same amount: 56 is something times 7.
- $8 \times 7 = 56$.
Answer: $8$
Common mistake
Writing the product in the box. For $8 \times \square = 48$, some people write 384 or 56 because they think "the answer goes after the equals." But 48 is already on the right side. Check: put your number in the box and see if both sides match: $8 \times 6 = 48$ does.