Matlab: plotting magnitude and phase response
@Hal: Unlike MBaz, I realize you weren't asking for information on the theory of signal processing. But, rather, you were asking a Matlab implementation question. I only have an old version of Matlab so I couldn't try Anthony's code. (Anthony was smart to insert that zero-valued coefficient.) But to plot the frequency mag. and phase responses of a discrete version of your filter, try these:

figure(1), freqz([2,0,3.125],[1,-0.9,0.81]);


or

freqz([2,0,3.125],[1,-0.9,0.81], 512, 'whole');