We started off by seeing that the sqaure was going to be worth 2 more than the triangle(20-18), so were able to with trial and error find how much the circles were worth. After working out the worth of the triangle, circle and square; we could easily figure out what the worth of the hexagon was.
================================================================================
----------------------------------SPOILERS--------------------------------------
================================================================================
Square = 8
Circle = 4
Triangle = 6
Hexagon = 7
================================================================================
--------------------------------------------------------------------------------
================================================================================