Puzzle ZGOE 

Workings

Hint
Consider the number of sides involved.

Answer
Square.

Square the first shape's sides and the remaining shapes add up to this number.

E.g.

   triangle pentagon square
      >> 3x3 = 5 + 4
   square hexagon hexagon square
      >> 4x4 = 6 + 6 + 4

Therefore:

   hexagon octagon octagon octagon octagon square
      >> 6x6 = 8 + 8 + 8 + 8 + 4