In the current state, the same set of material particles forms a tetrahedron of some other shape, and the forces acting on all faces are in the direction of x1 , with the forces on faces a1 , a2 , a3 being of magnitudes 4, 5, 6, respectively. calculate the force on face a. calculate the nominal stress tensor.

you'd use the equation that establishes a relationship between  energy  and  wavelength, which looks like this

e=h⋅cλ, where

λ  - the wavelength of the photon; c  - the speed of light, usually given as  3⋅108m/s; h  -  planck's constant, equal to  6.626⋅10−34j s

plug the value you were given for the energy of the photon in the above equation to get the wavelength

e=h⋅cλ⇒λ=h⋅ce

λ=6.626⋅10−34js⋅3⋅108ms−1.257⋅10−24j=1.581⋅10−1m