The value of b2 is _________

Question

The velocity field on an incompressible flow is given by

       V = (a₁x+a₂y+a₃z)I + (b₁x+b₂y+b₃z)j + (c₁x+c₂y+c₃z)k

       Where a_{1 = 2} and c₃ = –4. The value of b₂ is __________.