For this puzzle you have to figure out the centre number of the large tan triangle. To do this, look at the two triangles on the left. You will notice that in order to get the number in the centre you will have to add the top number and the leftmost number of the triangle. Then subtract the right most number from this sum. So:
15 + 21 - 4 = 32And 9 + 21 - 8 = 22Therefore 24 + 37 - 6 = 55
Enter 55 into the tan triangle to obtain the glowing key. Use it to escape this level.