I'm having trouble understanding one part of the proof I'm studying.

The matrix $\displaystyle \left[ \begin{array}{ccc} a & b & c \\ d & e & f \end{array} \right]$ is full-rank.

Now we suppose that this

$\displaystyle \left[ \begin{array}{cc} a & b \\ d & e \end{array} \right]$

is the full-rank part.

Why can we do this?

Thank you for your help, this is really confusing me.