Answer
See answers.
Work Step by Step
See the figures on pages 820 and 821.
Spontaneous emission: an atom is in an excited state, with higher energy than the ground state. The atom spontaneously (that is, on its own with no external stimulus) drops down to a lower energy level. To conserve energy, a photon is emitted with energy $\Delta E$.
Stimulated emission: the atom is in an excited state of an atom, which is metastable. A photon with the correct energy $\Delta E$ interacts with the excited atom. This causes (i.e., stimulates) the electron to immediately drop down to the lower energy level (it would have taken longer without the external stimulus, because the excited state is metastable).
At the end, there are two photons of the same energy $\Delta E$: the original one, and the emitted one.
