I watched an instructor write this question 3(x-7)=6, he then solved the Parentheses to get 3x-21=6, but here is where I was lost, he then added 21 on each side to get 3x=27, then divided each side by 3 to get x=9. Now, I always thought you you did everything in this order: Parentheses, Exponents, multiply/divide, then add/ subtract...but when I do this to this question, I got x-21=2, then x=23...can someone explain why he added/subtracted, before multiplied/divided, when trying to find the value of 'x'?
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analysis - Injection, making bijection
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