A uniform magnetic field B has constant strength b teslas in the z-direction [i.e., B=⟨0,0,b⟩ ]

(a) Verify that A=(1/2)B×r is a vector potential for B, where r=⟨x,y,0⟩
(b) Calculate the flux of B through the rectangle with vertices A, B, C, and D. A=(5,0,7) B=(5,3,0),
C=(0,3,0) D=(0,0,7) F=(5,0,0)

calculate flux B. So B is the vector field...? I'm also not sure how to use the divergence theorem on this problem.

Question

A uniform magnetic field B has constant strength b teslas in the z-direction [i.e., B=⟨0,0,b⟩ ]

(a) Verify that A=(1/2)B×r is a vector potential for B, where r=⟨x,y,0⟩ 
(b) Calculate the flux of B through the rectangle with vertices A, B, C, and D. A=(5,0,7) B=(5,3,0),
C=(0,3,0)  D=(0,0,7) F=(5,0,0)

calculate flux B. So B is the vector field...? I'm also not sure how to use the divergence theorem on this problem.