two samples of data determine if u can be 99% sure that company A makes more money than Company B

Co A Co B

mean 55,000 52,000

Std Dev 5,000 3,000

Sample size 200 400

I started out with z= 55,000-52,000

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V 5,000(2) + 3,000(2)

--------- ---------

200 400

Then, I get stuck?????? What do I do next to arrive at my answer?