well, we know there are 180° in π radians, so. how many radians in -36°?
[tex]\bf \begin{array}{ccll}
de grees&radians\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
180&\pi \\
-36&r
\end{array}\implies \cfrac{180}{-36}=\cfrac{\pi }{r}\implies r=\cfrac{-36\cdot \pi }{180}[/tex]