Which signed representation has to be taken?
5 bits required to specify 32 registers for register operand. So, 2 register operands 10 bits. 6 (⌈log249⌉) bits required for opcode(since 49 instructions). So (10 + 6 = ) 16 bits required. So left no. of bits = 24 - 16 = 8 to represent immediate operand.
As the operand is signed integer, we can represent -27 to 27 - 1. So, the max value will be -27 = -128.
Signed integers are stored in 2's complement. If you have doubt regarding signed integers, see this link.