In $2's$ complement representation for signed numbers, positive numbers are represented as is in binary and negative numbers are represented in their $2's$ complement form.
$2688 = 2048 + 512 + 128 = 0000\;1010\;1000\;0000.$
$2's$ complement of $2688 = \underbrace{1111}_{F}\;\underbrace{0101}_{5}\;\underbrace{1000}_{8}\;\underbrace{0000}_{0} = (F580)_{16}$