Question: system with delay characteristic equation

Dear all

have a nice time. 

I have a system of nonlinear differential equation with continuous delay.  I tried to find some books that define a strategy to compute the characteristic equation but unfortunattely the most of papers or books give directly the formula without any proof. 

I tried with maple, but unfortunattely no solution up to now. 

I hope find the strategy how find the characteristic equation of the attached system.

characteristic_equation.mw

thanks 

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