The angular momentum of the first disk was L_1 = I_1 omega_1 = 2 kg m^2 * 5 rad/s = 10 kg m^2 rad /s
the angular momentum of the second disk was L_2 = I_2 omega_2 = 2 kg m^2 * -11 rad/s = -22 kg m^2 rad /s, and the total angular momentum of the system was 10 + -22 = -12 kg m^2 rad /s. Note that after the disks are stuck the angular momentum is still the same.
After the two disks are stuck together, their moment of inertia will become 4 kg m^2, and their angular velocity will be -12 kg m^2 rad /s / 4 kg m^2 = -3 rad/s. The magnitude of the angular velocity will be 3 rad/s.