Experiments are performed with ultracold neutrons having velocities of 7.54 m/s. (a) What is the wavelength (in nm) of such a neutron

     λ = 52.5 nm

Explanation:

De Broglie's duality principle states that all matter has wave and particle characteristics, being related by the expression

          p = h / λ

where the moment

          p = mv

          λ = h / mv

let's calculate

        λ = 6.63 10⁻³⁴ / 1.675 10⁻²⁷ 7.54

        λ = 5.25 10⁻⁸ m

Let's reduce anm

       λ = 5.25 10⁻⁸ m (10⁹ nm / 1m)

       λ = 52.5 nm

answer: 31, 5556. 952 seconds