Hey there!:
Molar mass BaS => 169.39 g/mol
Number of moles:
n = mass of solute / volume ( L )
n = 12.5 / 169.39
n = 0.0737 moles of BaS
Volume of solution:
M = n / V
0.216 = 0.0737 / V
V = 0.0737 / 0.216
V = 0.3412 L
Now converts the volume to mL :
0.3412 L * 1000 => 341.2 mL
Hope that helps!