An Electron in an Infinite Potential Well (A Box)makes a Transition

Short Answer

An electron in an infinite potential well (a box)makes a transition from the n = 3 level to the ground state and in so doing emits a photon of wavelength 20.9 nm.(c = 3.00 × 10⁸ m/s, ,m_el = 9.11 × 10^-31 kg)
(a)What is the width of this well?
(b)What wavelength photon would be required to excite the electron from its original level to the next higher one?