You need to translate the English into math.
You are asked to find the mass of bricks so that is your unknown.
Let X = the mass of bricks.
The mass of bricks and the load = 11600.
So the empty lorry (the British word for a truck) is 11600 - X.
If the bricks weighed 100 kg, then the truck would weigh 11500, right?
The bricks weigh 3 times as much as the empty truck.
So 3 times the weight of the truck equals the weight of the bricks.
3(11600-X) = X
Now solve the equation by distributing 3 over the contents of the parentheses. Then get all X's on the right side. Then divide both sides by the coefficient in front of the X on the right and you'll determine what X equals.