How to find covariance matrix from correlation if mean is not given?

Question

I'm given autocorrelation function of gaussian random process: $$ R_x(\tau) = 3e^{|-\tau/3|} $$ Now I should find covariance matrix. I know the formula and solutions, where $$ C_{xx} = R - E[X]E[Y] $$ But in this example I don't know how to find E[X] and E[Y] from $$ R_{xx} $$