Might be -14.
The pattern "1111 0010" might be a good choice for "negative fourteen". In the picture, the only good choice for "Mystery Number" is negative fourteen.
For every bit pattern of N bits there is a corresponding bit pattern of N bits which produces an N-bit zero when the two patterns are used as operands for the binary addition algorithm. Each pattern can be thought of as representing the negative of the number that is represented by the other pattern.