from 170 to 97, there's a difference of 73.
so hmm if we take 170 to be the 100%, what is 73 in percentage off of it?
[tex]\bf \begin{array}{ccll}
amount&\%\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
170&100\\
73&p
\end{array}\cfrac{170}{73}=\cfrac{100}{p}\implies p=\cfrac{73\cdot 100}{170}[/tex]