You forgot to include the known characteristics of the solution.
I searched them and copy here:
volume: 1.000 liter
M = 0.0190 M
Now, you can start with the definition of mass percent.
mass percent = (grams of solute / grams of solution) * 100
grams of solute are obtained from the molar concentration:
M = (number of moles of solute) / (volume of solution in liters)
where number of moles = (grams) / (molar mass)
=> M = (grams of solute / molar mass of solute) / (volume of solution in liters)
=> grams of solute = M * (volume of solution in liters) * (molar mass of solute)
And density = (kg of solution / volume of solution in liters) =>
kg of solution = density * volume of solution in liters
grams of solution = density * (volume of solution in liters) * 1000 g/kg
=> mass percent = M * (volume of solution in liters) * (molar mass) / (density * volume of solution in liters * 1000 g/ kg) * 100
=> mass percent = M * molar mass * 10 / density
now replace the values known:
M = 0.0190 mol / liter
density = 1,000 kg / liter
molar mass of Ba(NO3)2 = 137.327 g/mol + 2*14.007 g/mol + 2*3*15.999 g/mol = 256.335 g/mol
=> mass percent = 0.0190 mol/liter * 256.335 g/mol * 10 kg/ g / (1.000 kg/liter)
=> mass percent = 48.7%