Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: if ( 5 - x ) / ( 7 + x ) = x , what is the value of x ^ 2 + 8 x - 5 ?
0
-15/4.