Question 1198927
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Go with the response from @MathLover1; the response from @Theo is mathematically incorrect.<br>
It is true that, in everyday usage, to find the difference between two number you subtract the smaller from the larger -- resulting in a positive answer for the difference.<br>
But in formal mathematics, the "difference between A and B" is always A-B, even if the result is negative.<br>
For the expression in your post....<br>
"a number decreased by..." --> x- ...<br>
"the difference between 9 and the number: --> 9 - x<br>
"a number decreased by the difference between 9 and the number: --> x - (9 - x) = x - 9 + x = 2x - 9<br>