Hey there!:
Molar mass LiBr = 86.845 g/mol
Mass of solute = 40 g
Volume of solution = 7.5 liters
Number of moles of solute:
n = mass of solute / molar mass
n = 40 / 56.845
n = 0.0007036 moles of solute
Therefore:
Molarity = moles of solute / volume
Molarity = 0.0007036 / 7.5
Molarity = 0.0000938 M
Hope that helps!