Given that:
[tex]mPQ=360\text{\degree}-210\text{\degree}[/tex]You get that its measure is:
[tex]mPQ=150\text{\degree}[/tex]The exercise provides the formula for calculating the length of PQ:
[tex]length\text{ }PQ=\frac{mPQ}{360\text{\degree}}\cdot(2\pi r)[/tex]Where "r" is the radius of the circle.
Knowing that:
[tex]\begin{gathered} r=30 \\ \pi\approx3.14 \end{gathered}[/tex]You can substitute values into the formula and then evaluate:
[tex]length\text{ }PQ=\frac{150\text{\degree}}{360\text{\degree}}\cdot(2)(3.14)(30)[/tex][tex]length\text{ }PQ=78.5\text{ }units[/tex]Hence, the answer is: Option C.