[ Eq(x3, k9 - x4), Eq(x4, k9*2*k5*k8/(k7*x1 - k5 + k6*k7*x1 + k8*k5 + ((k7*x1 - k5 + k6*k7*x1 + k8*k5)**2 - 4*(k7*x1 - k5)*k5*k8)**(1/2))) ]