2^192(256-31) + 2^n
2^192*225 +2^n ..........(1) , now let (1) be a perfect square and it is equal to m^2
m^2 - (2^96*15)^2 =2^n ; this is split into 2^n = (m + 2^96*15) * (m - 2^96 * 15) ................(2)
let n = 2A + B ; and substitute in (2)
gives 2^(A+ B) = m + 2^96*15 ..........(3) and 2^A = m - 2^96 *15 .......( 4)
solving equations 3 and 4 ; we have A = 97 and B = 4
therefore n = 2*97 +4 = 198
n = 198 gives the expression a perfect square.