In case of signed Magnitude Representation the range is from $-(2^{n-1}-1)$ to $2^{n-1}-1$
Min number that can be represented in this system is $-(2^{n-1}-1)$
The max number that can be represented in this system is $2^{n-1}-1$
In the case of 2's complement no system, the range is from $-(2^{n-1})$ to $2^{n-1}-1$
Min no that can be represented in this system is $-(2^{n-1})$
Max no that can be represented in this system is $2^{n-1}-1$
As they said 2 Bytes = 16 bits
we can use max no that can be represented here which is $2^{n-1}-1\implies 2^{16-1}-1=2^{15}-1$