Since you know that a square has 4 equal sides then you know the small squares are 4cm on all sides and the big squares are 9cm on all sides.
If 2 and a partial sm sq are next to a 9cm sq then knowing 4*2=8 you can figure out that the other square is split as 1cm and 3cm(1+3=4), 3cm of which is lined up with the width (4cm)of another sm sq which would equal 7cm, these 2 sm sqs line up with the unknown sq whose sides would in turn also be 7cm.
