How to find the combined variation without the observations?

#1
hello,
i have a question i have been struggling finding with for the last week and i would love to get some help (i have asked friends, looked online and tried really hard by myself). they ask me to get the combined variation of the 2 groups, and provide the following data:

group a -
mean: 300
Variance: 50
total number of people in the group: N

group b-
mean: 200
Variance: 0
total number of people in the group: 3*N

any help will be highly appreciated,
thank you!
 
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