so if given a covariance matrix I can find the eigenvalues and move forward from there... but I seem to have trouble with the step before if I am given a data set and am told to create the covariance matrix. Looking at the notes I see the formula: $$cov(x) = \frac{1}{n-1}\sum(x_i - \bar{x})(y_i -\bar{y}) $$

I'm not too sure what to do with this formula and was hoping you can tell me how.

Data Set:

X1 | X2 ---|--- 3 | 7 2 | 4