(ans is 2.16 × 103 nm, i need the step, thankyou so much) Calculate the wavelength, in nanometers, of the light emitted

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(ans is 2.16 × 103 nm, i need the step,thankyou so much)
Calculate the wavelength, in nanometers, of the light emitted bya hydrogen atom when its electron falls from the n = 7 to the n = 4principal energy level. Recall that the energy levels of the H atomare given by En = -2.18 ×10-18 J(1/n2)