nonlinear headache

could some expert recommend a Liapunov function that might help control this nonlinear differetial equation:
y = x * (1 - K * F(s) * |x|^2)
here F(s) = 1/(1+sb), represents a 1st order low pass filter with time constant 1/b, K is a "small" positive number; both b and K are unknown.
I need a control law x = C[u] such that y = u.
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