Molarity (M) is calculated as moles of solute/Liters of solution (mol/L). We can use the information given to find the number of moles of KBr used to make the 0.63 M solution and then convert the moles to grams.
0.630 mol/L = ? moles / 0.415 L
0.630 mol/L × 0.415 L = 0.261 mol KBr
Notice how we changed mL to L so that the units cancel out to give you the answer in moles. Now, use your periodic table to find the molar mass of KBr to be 119 g/mol. Let's use that to convert the moles to grams.
0.261 mol KBr × 119 g/mol = 31.1 g KBr
