Lay two sheets sideways, one above the other,
with their 28cm sides together, like this:



This makes a rectangle which is 2x21.5cm 
or 43cm tall. So AB = 43cm.

Now place a third sheet upright on top of those
like this:



Now since AB = 43cm, and BC = 28cm,
then AC = AB - BC = 43cm - 28cm = 15cm.

Edwin