Given mass of solute = 340 gram
Molar mass of solute(MgBr2) = 184 g
Number of moles = Given mass / Molar mass = 340 / 184 = 1.85 moles
Now, Molarity = Number of moles / solution (in litres)
So, M = 1.85 / 8
M = 0.23 M
In short, Your Molarity would be: 0.23 M
Hope this helps!