Ответ:

123_N = 93_(N+2)

1*N^2 +2*N^1 +3*N^0 = 9*(N+2)^1 + 3*(N+2)^0

N^2 +2N +3 = 9N+18 +3

N^2 +2N -9N -18 = 0

N^2 -7N -18 = 0

D = 49 +18*4 = 121

N = (7-11)/2 = -2

N = (7+11)/2 = 9

Ответ: 9.