Find each interior angle of a regular octagon in radians…

One radian is the angle at the centre of a circle when the arc is as long as the radius, so $\theta = \frac{s}{r}$. A full turn is $2\pi$ rad, and $\pi$ rad $= 180^\circ$. Degrees into radians: multiply by $\frac{\pi}{180}$. Radians into degrees: multiply by $\frac{180}{\pi}$. The interior angles of a polygon with $n$ sides add up to $(n - 2)\pi$.