Nathan B. answered • 01/16/20

Elementary and Algebraic skilled

In the order of operations to find a number, multiplication and division take place before addition and subtraction. When we are trying to isolate an *unknown *number, however, we perform operations in reverse to apply them to the opposite side of the equation symbol. So in this case, we take that -9, and add it to both sides:

6 * [ ] - 9 + 9 = 21 + 9

6 * [ ] = 30

From here, you can ask yourself "six times what equals thirty?"

Otherwise you can divide, much like what we did earlier:

6 * [ ] ÷ 6 = 30 ÷ 6

[ ] = 30 ÷ 6

30 ÷ 6 = 5