I have to find the state equation from the circuit, the problem is, on the node with red marker. Here is what I've done
\$u = I_C+I_R\$
\$u = C\frac{dV}{dt} + \frac{V}{R}\$
and if the value from components is included, then
\$u = \frac{dV}{dt} - V\$ (because of negative resistance)
is this right? based on the solution given, it should be
\$u = \frac{dV}{dt} + V\$
is the negative in resistance is ignored?