159320

Circle: 4
Square:8
Triangle:6
Hexagon:7

?: 21

I worked this out by using trial and error on the top line (triangle, square, square, triangle). After working that out I could work out the rest by seeing that the 3rd (across) line had three circles and one triangle. I could work out that 18-6=12. Then I divided by 3 (three circles) and the answer was 4. Once I had the circle, square and triangle the hexagon came easily: 8+8=16 (squares) 30-16=14 and then 14 divided by 2 equals 7 because there are two hexagons.