inverse matrix

cofactor of B =
[-1 -2]
[-3 1]

adjoint of B
= transpose of B =
[-1 -3]
[-2 1]

Therefore, B^-1 =
(1/det B) x (adjoint of B)
= (1/-7) x [-1 -3; -2 1] =
[1/7 3/7]
[2/7 -1/7]

If you have any problems, please feel free to ask.