While you arrived at the correct answer, saying "multiply both sides of the equation by m" is actually misstated because "1/3m" is not "1 / 3m" it is actually "1m / 3" or "m/3"
Thus the equation would actually read,
m/3 + 2m/3 = -1
The denominator for both fractions is 3, thus the equation becomes,
3m/3 = -1
m=-1
Same answer, just the equation and method for solution worded more accurately.
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