The scale is balanced so both sides need to weigh the same amount.
On the left side you have:
6 squares, 3 circles and 2 triangles
On the right side you have:
3 squares, 2 circles and 5 triangles
Sunday equal amounts of each from both sides you would have:
3 squares and 1 circle in the left and 3 triangles in the right.
So we know the 3 squares and 1 circle = 3 triangles:
3 squares x 2 grams = 6 grams
1 circle x 3 grams = 3 grams
6 + 3 = 9 grams
9 grams / 3 triangles = 3 grams per triangle.,