I don't under stand what is wrong.
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Answer:
In the first step, there is an issue when distributing the -3 to the terms inside of the parentheses. When multiplying the -3 to the -5, you would get an answer of +15, not -15.
The full correct process is:
(4m + 9) - 3(2m - 5) = 4m + 9 - 6m + 15
= 4m - 6m + 9 + 15
= -2m + 24