You added 40 g sugar to the solution.
Step 1. Calculate the mass of the first solution.
Mass1 = 120 mL × (1.22 g/1 mL) = 146.4 g
Step 2. Calculate the mass of the second solution.
Volume 2 = 120 mL + 15 mL = 135 mL
Mass 2 = 135 mL × (1.38 g/1 mL) = 186.3 g
Step 3. Calculate the mass of the sugar.
Mass of sugar = mass2 – mass1 = 186.3 g – 146.4 g = 40 g