{ 1*k21*x1 + 1*k21*x2 + 1*k21*x3 + 1*k21*x4 = k21*k22,
  1*k21*x5 + 1*k21*x6 = k21*k23,
  1*k21*x5 + 1*k21*x9 + 1*k21*x10 = k21*k24,
  1*k21*x7 + 1*k21*x8 = k21*k25 }