Molarity has the definition of mols/liter of solution.
You want 0.750 M; therefore, weigh 0.750 mol x (molar mass/1 mol) Ba(OH)2, dissolve it in some water and make the final volume to 1.00 liter.(Note I did NOT say to add 1.00 L of water. Adding 1 L of water to Ba(OH)2 results in a volume that is more than 1.00 liter of solution; hence, we dissolve the solute in a little solvent and make the final volume to the desired level.
Look up the molar mass of Ba(OH)2, or calculate it.
my answer is 128.495 is that right Dr.Bob?
I have 171.342 for the molar mass of Ba(OH)2 but I don't know the reliability of those values I added together. The numbers you used may be more up to date. Anyway, then 0.750 x 171.342 = 128.506 which would round to 128 to three significant figures (0.750 has three s.f.). But I think the important point your prof is trying to impress on the class with this problem is to give directions for preparing. So the directions would go something like this. Weigh on an analytical balance 128.5 g Ba(OH)2, dissolve in water, and make the final volume to 1.00 liter to prepare 1.00 L of a 0.750 M Ba(OH)2 solution.
thank you Dr. Bob