Question 1200411
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x = starting unknown number
x-8 = subtract off 8
3(x-8) = multiply the result by 3
<font color=blue>3(x-8) = 21</font> is the equation to set up


Let's isolate x. 
To do so, we follow PEMDAS in reverse to undo what happens to x.
The last thing that happened was "multiply by 3", which means we undo that by dividing both sides by 3
3(x-8) = 21
3(x-8)/3 = 21/3
x-8 = 7


Then we add 8 to both sides to undo the "minus 8" aka "subtract 8".
x-8 = 7
x-8+8 = 7+8
x = 15


Here are all of the steps in one big block
3(x-8) = 21
3(x-8)/3 = 21/3
x-8 = 7
x-8+8 = 7+8
x = 15


Here is another route we could take through the use of the distributive property
3(x-8) = 21
3x-24 = 21
3x-24+24 = 21+24
3x = 45
3x/3 = 45/3
x = 15


Check:
Starting number = 15
Subtract off 8 to get 15-8 = 7
Triple the result: 7*3 = 21
The answer is confirmed.


Answer: <font color=red>15</font>
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