How to get arc length when central angle is 139

So, this is a really good video to help you. I hope you get the answer, if you still don’t- I’ll answer your question in the comments. https://youtu.be/lBOy62v-9Uk

The radius  of the hemisphere is:
V=πh²(r-h/3)
because the volume of the hemisphere doesn't change , we measure the change in the volume in relation to the change in height:
dv/dt=π(h²-(1/3 dh/dt))+2h(r-h/3)dh/dt)
dv/dt=π(-h²/3+2hr-2h²/3)dh/dt
dv/dt=π(2hr-h²)dh/dtdv/dt=-2 ft³/h, height is 8 ft and radius of the hemisphere is 9 ft
-2=π(2h*9-h²)dh/dt
-2=π(2*8*9-8²)dh/dt
-2=π(80)dh/dt
dh/dt=-2/(π*80)
dh/dt=-0.008