Two waves are interfering with each other. the first wave has an amplitude of +2 cm, and the second wave has an amplitude of -1 cm. when these two waves superpose, what is the amplitude of the superposition? cm +1 +3 -3 no change

There's no such thing as a wave with negative amplitude.

If these two waves have the same frequency and meet in phase,
then the amplitude of the superposition is  3 cm.

If they have the same frequency and meet out of phase,
then the amplitude of the superposition is  1 cm.

If they have the same frequency and meet at some intermediate phase, then
the amplitude of the superposition is something between 1 cm and 3 cm.

If the two waves do not have the same frequency, then their superposition
is a complicated combination of both frequencies and both amplitudes.