Chapter 4 - Vector Spaces - 4.6 Exercises - Page 239: 9

$\dim ColA = 2$

Given the null space of a 5 x 6 matrix, A, is 4-dimensional; The dimension of column space is equal to its rank. $Rank A=n-dim\,Nul A$ $n=6$ and $dim\,Nul A=4$ Therefore $Rank A=6-4=2$ $\dim ColA =Rank\,A= 2$