markov&availability

hi,
i made a markov model of a maintenance system with 2 treshold values
and 3 types of maintenance activities. my first state is P(0,0).
i write all my states in form of P(0,0), and then availability(A)
function(as we know the sum of all states is 1). i want to find the
mean time between inspections (let say x) which max A (as dA/dx).

is there any way to find the optimal x without giving any numerical
values to other constraits(treshold values, mean maintenance times,
mean inspection time and x). and how can i plot the A versus x graph
as a result of dA/dx.

thanks..
.