Hey there!:
Molar mass AgNO3 = 169.87 g/mol
Number of moles:
moles of solution = mass of solute / molar mass
moles of solution = 18.7 / 169.87
moles of solution = 0.110084 moles of AgNO3
Volume in liters:
250.0 mL / 1000 => 0.25 L
Therefore:
Molarity = moles of solution / Volume of solution ( L )
Molarity = 0.110084 / 0.25
=> 0.440 M
Hope that helps!