First of all, I started on attempting to work out how much the Red Circle is worth. This is the easiest target to start with, as there is 3 circles in 2 different rows. I did trial and error, and eventually found out that the circle is worth 4. It shows that that particular row is worth 18. So I took off the amount of the three circles off the 18 (Three circles x 4 = 12) 18 - 12 = 6. After that I knew that the final symbol is worth 6, which was the triangle. Now I know that the triangle is worth 6, and the circle is worth 4.
Once I had worked out that, it was rather easy to finish off the rest and find that the square is worth 8, and the hexagon is worth 7. To make sure that my method is correct, I checked my answers. Also, the answer where the question mark is, is worth 21. 2 x circle + 1 x triangle + 1 x hexagon (8 + 7 + 6 = 21).