First write out a balanced chemical equation:
SrBr2 + Cl2 --> SrCl2 + Br2
This tells you that one mole of SrBr2 will produce one mole of Br2 gas.
Now one mole of gas at STP is equal to 22.4 liters.
75 L/22.4 L/mole = 3.34 moles of Br2 produced.
So you need 3.34 moles of SrBr2.
Molar mass of SrBr2 = 247.42
So you need 3.34 moles * 247.42 g/mole = 828.86 g of SrBr2, assuming 100% conversion rate.