Let $B$ be the matrix defined by

$$B = \left[\begin{array}{cccc} -4 &6 &1 &5\cr 7 &-5 &1 &-6\cr -1 &3 &1 &2\cr 4 &-2 &1 &-3 \end{array}\right]$$

Find $\mbox{rank}(B)$ by either computing a basis for the column space of B or by computing a basis for the row space of B.

$\mbox{rank}(B) =$