cutoff

Let the following transfert function : C(p) = 0.5926 \frac{1+1.205p}{1+0.4233p} = K \frac{1+a \cdot \tau p}{1 +p \cdot \tau p} with a = 2.848, \tau = 0.4233, K = 0.5926 the marge of phase of this corrector is 28.7° i want to find the cutoff frequency of this correct, the answer was already...