[tex]a=00110111 = 00110_2 \times 2^{111_2} = 6 \times 2^{-1} = 3[/tex]
[tex]b=11011000 = 11011_2 \times 2^{000_2} = -(0100_2 + 1) =-(4+1)=-5[/tex]
[tex]a+b=-2 = -1 \times 2^{001} = -2 \times 2^{0} = -4 \times 2^{-1}[/tex] etc.
[tex]a+b=-2 = 11111001 = 11110000 = 11100111 = ...[/tex]
Hard to select the correct answers without seeing the choices. Let's check a couple,
[tex]11111001 = -(00000+1) \times 2^{1} = -2 \quad\checkmark[/tex]
[tex]11110000 = -(00001 + 1) \times 2^{0} = -2 \quad\checkmark[/tex]
[tex]11100111 = -(00011 + 1) \times 2^{-1} = -4/2= -2 \quad\checkmark[/tex]