5 The diagram shows a sector \(O A B\) of a circle with centre \(O\) and radius \(r \mathrm {~cm}\).
\includegraphics[max width=\textwidth, alt={}, center]{30ccdbe9-0c91-4011-a3f9-3ce01862215d-10_346_360_360_824}
The angle \(A O B\) is \(\theta\) radians.
The area of the sector is \(12 \mathrm {~cm} ^ { 2 }\).
The perimeter of the sector is four times the length of the \(\operatorname { arc } A B\).
Find the value of \(r\).
[0pt]
[6 marks]
\includegraphics[max width=\textwidth, alt={}]{30ccdbe9-0c91-4011-a3f9-3ce01862215d-10_1533_1712_1174_150}