Can anyone help me understand this?

I don't understand why the BC turn into a 0. Can someone explain that to me?

 

Its already written their, the term BCAB' turns into 0 as for identity AA' = 0 in case of BCAB' the BB' = 0 ,So BCAB' = ACBB' = AC * 0 = 0
You need to know DeMorgan’s theorem and Boolean arithmetic

Good Luck

 

That make more sense to me. Thanks...