The square is 2 more than the triangle. This must be because on the second column there are three squares and one triangle and the squares have to be the same number. So I choose 8 to be my number as I had work out that the triangle had to be two less than the square. So the triangle is 6
Then the hexagon had to be more than the triangle as when there was three squares and one hexagon the answer was bigger than it was with one triangle. But then the hexagon couldn't be bigger than the square so we were left with 7 to be the hexagons number.
Then to work out the circle I took the third row down and saw that the answer was 18, but that row had a triangle which know is 6 so 18-6=12. The we had 3 circles so you have to divide 12 by 3 which gave us the answer of 4.
So these are the answers:
Square is 8
Triangle is 6
Hexagon is 7
Circle is 4
So now all you have to do is add the column that you need.
Which is Triangle+Hexagon+Circle+Circle=6+7+4+4=21
So 21 is your answer.