[ Eq(Derivative(x1, t), ((-1)*k12*k1*x4*x1 + 1*k12*k2*x2 + 1*k12*k4*x3)/k12), Eq(Derivative(x2, t), (1*k12*k1*x4*x1 + (-1)*k12*k2*x2 + (-1)*k12*k3*x2)/k12) ]