Precalculus (10th Edition)

$\frac{13\pi}{9}\text{radians}.$
To convert from degrees to radians, the answer in degrees must be multiplied by $\frac{\pi}{180^\circ}$. Therefore: $260^\circ=260\cdot\frac{\pi}{180^\circ}\text{radians}\\=\frac{13\pi}{9}\text{radians}.$