Binary subtraction with numbers in 2's complement
You cannot represent –173 using 8 bits, since the range is –128 to 127.
To subtract two numbers in two's complement, you use the identity $a - b = a + (-b)$. In your case, $a = 10100110$, $b = 01010011$, and $-b = 10101101$. Adding $a$ and $-b$, there is overflow, so you know that the number is outside the range.