$($a$)$ Convert the following Hex numbers to binary and add them. Express your final answer in HEX

$2A7E+45CB$

$($b$)$ Write down the $2\text{'}$s complement of the following binary number

$10110000$

$($c$)$ Convert the following numbers to binary and evaluate using the $2\text{'}$s complement method. Leave your answer in binary form.

$68-145$