اگر $f(x)=\left\{ \begin{matrix}
{{x}^{2}}+ab\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x\ge 3 \\
-x+2a\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,1\le x\le 3 \\
2x+1\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x\le 1 \\
\end{matrix} \right.$ یک تابع باشد، $a-b$ کدام است؟