Solution:First letter : W - 2 = U , U - 2 = S, S - 2 = Q, Q - 2 = O
Second letter : X - 2 = V, V - 2 = T, T - 2 = R, R - 2 = P
Third letter : N - 2 = L, L - 2 = J , J - 2 = H, H - 2 = F
Fourth Number : 13 + 6 = 19, 19 + 6 = 25, 25 + 6 = 31, 31 + 6 = 37
So, ? = OPF37