On the left side we have a triangle and two squares and on the right side we have 3 triangles an a square, since they are on a balance, they should weight the same.
On the left side we have 1 gram (because of the triangle) an two squares, so we can write 1 + 2x (x is the weight of a square)
On the right side we have 3 grams ( because of the three triangles) and one square, so we can write 3 + x (x is the weight of a square)
Since they are both the same:
1 + 2x = 3 + x
Solving for x:
1 + 2x = 3 + x
2x - x = 3 - 1
x = 2
Answer:
A square weights 2 grams