Use the formula:
E_n = -R_H(1/n^2)En=−RH(1n2)
where :
R_H = "Rydberg constant" = 2.18 * 10^(-18)JRH=Rydberg constant=2.18⋅10−18J,
n = "the energy level"n=the energy level
For n = 4n=4:
E_4 = -2.18 * 10^(-18)J(1/4^2) = -1.3625 *10^(-19)JE4=−2.18⋅10−18J(142)=−1.3625⋅10−19J
For n = 2n=2
E_2 = -2.18 * 10^(-18)J(1/2^2) = -5.45 *10^(-19)JE2=−2.18⋅10−18J(122)=−5.45⋅10−19J
Then to find the energy of the photon, you just calculate the change in energy.
DeltaE = E_"initial" - E_"final"
DeltaE = (-1.3625 *10^(-19)J) - (-5.45 *10^(-19)J)
DeltaE = 4.0875 * 10^(-19)J
Since you only have 3 significant figures, the answer would be:
4.09 * 10^(-19)J