I am unsure of how to solve this problem the correct answer

I am unsure of how to solve this problem the correct answer was 15 eV but I keep coming up with 20 eV insted.

Solution

energy of particle in a box = n^2h^2/(8ma^2)

E1 = 5 ev = h^2/(8ma^2)

E2 = 2^2* h^2/(8ma^2)   = 20 eV