We have,
\begin{align*}
\zerovector
&=\vect{x}-\vect{x}&&\knowl{./knowls/property.AI.knowl}{\text{Property AI}}\\
&=\alpha\vect{x}-\vect{x}&&\text{Hypothesis}\\
&=\alpha\vect{x}-1\vect{x}&&\knowl{./knowls/property.O.knowl}{\text{Property O}}\\
&=(\alpha-1)\vect{x}&&\knowl{./knowls/property.DSA.knowl}{\text{Property DSA}}
\end{align*}
So by Theorem SMEZV we conclude that $\alpha-1=0$ or $\vect{x}=\zerovector$. However, since our hypothesis was for every $\vect{x}\in V$, we are left with the first possibility and $\alpha=1$.
There is one flaw in the proof above, and as stated, the problem is not correct either. Can you spot the flaw and as a result correct the problem statement? (Hint: Example VSS).