A cantilevered beam is loaded as shown. The cross section at the wall is shown, with points of interest $A($at the top$),B($at the center$),$ and $C($midway between A and $B).$

wnere $r=3KN$

NOTE: This is a multi$-$part question. Once an answer is submitted, you will be unable to return to this part.

If the length of the beam is varied from longer to shorter, the critical stress element will switch from point $A$ to point $B.$ At what length does this happen?

This will happen at $mm.$