The mass is 3 mol of calcium bromide.

Answer: 600 g.
Explanation of the solution to the problem: to solve this problem, you must use the following formula:
m = n * M, where n is the amount of calcium bromide substance (from the condition of the problem n (CaBr2) = 3 mol), m is the mass of calcium bromide, M is the molar mass of calcium bromide (from the periodic table of elements of D.I.Mendeleev: M (CaBr2) = 40 + 2 * 80 = 200 g / mol).
Substituting the values known from the condition of the problem into the formula, we obtain:
m (CaBr2) = n (CaBr2) * M (CaBr2) = 3 * 200 = 600 g.