[ Eq(x3, k7 - x4),
  Eq(x4, k7*2*k3*k6/(k5*x1 - k3 + k4*k5*x1 + k6*k3 + ((k5*x1 - k3 + k4*k5*x1 + k6*k3)**2 - 4*(k5*x1 - k3)*k3*k6)**(1/2))) ]