Nathan B. answered • 01/16/20
Tutor
5
(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
