need help with covariance matrices, seems really easy

I do not know covariance matrices at all. So I am trying figure out how to do this. I am running this is in matlab.

so z is a 2 X 3 matrix that looks like this

z = 4 2 5
1 8 9

and covariance of z equals
cov(z) = 4.5 -9 -6
-9 18 12
-6 12 8

can someone please explain how this came to be???????????

could someone just take the values and explain it step by step for atleast the first two values then i could probably figure out the rest.


thank you very much

the spacing didnt work like i typed well its

z =
4, 2, 5
1, 8, 9