Answer: 255
Step-by-step explanation:
With 9 bits, the largest number we can represent in 2's complement is (2^8) - 1, where 8 is the number of bits used to represent the magnitude of the number (since the leftmost bit is reserved for the sign in 2's complement).
Therefore, the largest number we can represent in 2's complement with 9 bits is (2^8) - 1 = 255 in decimal.