The weight of the square is 8 grams.
What is the weight of the square?
Let's define the variables:
x = weight of a triangle.
y = weight of a square.
We can see that we have equilibrium, so the total weight in both sides must be equal, then we can write the equation:
x + y + y = x + x + x + y
We know that x = 4 grams, then we can replace:
4g + 2y = 3*4g + y
4g + 2y = 12g + y
Solving this equation for y, we will get:
2y - y = 12g - 4g
y = 8g
The squares weight 8 grams.