Perpendicular point to two vectors

Hello, I'm struggling how to find A' vector given A and B vectors with direction from A to B that A' will be perpendicular to B. Below are screens of how I see it but can't find a way to actually calculate it.

Also, I was trying to find the middle cube that is most in front from the samples (in this case it would be 5th cube starting from A) and then trying to move the others cubes so tey aligns in line with that 5th cube giving me a straight line with a preserved relative position of each one so move them only one axis until aligned. But can't wrap my head around this one too...

Could anyone help me with that?

P.S. Sorry if the explanation is not good. It's hard for me to wrap it up in my head right now.