نمودار تابع با ضابطه $f\left( x \right)=\left\{ \begin{matrix}
ax+1\,\,\,\,\,\,\,;\,\,\,\,\,\,x\gt 2 \\
b\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,;\,\,\,\,\,\,\,x=2 \\
{{x}^{2}}-1\,\,\,\,\,\,\,\,\,\,;\,\,\,\,\,\,\,\,\,x\lt 2 \\
\end{matrix} \right.$ همواره پیوسته است. $a-b$ کدام است؟