Looking at the bottom two rows, the only differing shapes are in the 2nd column, therefore Square = Triangle + 2.
Then, looking at the first rows, we see the total of 28.
28 = 7x4.
If you make two of the 7's an 8, (Square) and balance it by making the remaining two 7's into 6's, (Triangle) we recive the following information.
Square = 8
Triangle = 6.
Back to the bottom two rows.
(Circle x 3) + (Square x 1) = 20.
20 - Square = 12.
12 divided by 3 = 4.
We now know ;
Square = 8
Triangle = 6
Circle = 4.
Finally, we look at the 2nd row down.
(Hexagon x 2) + (Square x 2) = 30
30 - (Square x 2) = 14
14 divided by 2 = 7
The full list of numbers becomes;
Square = 8
Triangle = 6
Circle = 4
Hexagon = 7.
Therefore, the missing column is 21.
(Circle x 2) + Hexagon + Triangle = 8 + 7 + 6
= 21
Done :D