This is a rating of winds on the Beaufort scale, which is described on wikipedia. Force 9 winds are considered a 'strong gale' and result in waves 23-32 feet high.
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There is only one set of data so there is only one mean. Therefore there is no standard error of the mean.
If your question refers to the Geometric Mean of 9 and 11 then the GM of any two numbers is √ab. Then the GM of 9 and 11 = √(9 x 11) = √99 = 9.9499 (5sf)
sqrt(9 * 16) = sqrt(144) = 12
6.0
If 6 numbers have a mean value of 9 then they must add to 54 (6*9) not 56. The question can not therefore be answered as the figures are incorrect.