Draw the Venn Diagram of (A intersect B)' 

A intersect B (without the brackets and complement symbol)
is JUST the area where A and B overlap.
By definition, the complement is the OPPOSITE of that, which means
That three areas are shaded (outside the circles, the 'just-A' area and the 'just-B' area)
and only one area ISN'T shaded ... the overlap between the two circles.