Circle has radius of 6cm.
Sector has arc length of 8.4cm.
The angle at the centre of the sector is ?

What is the angle at the centre of the sector? Please explain?
I only know how to calculate length of arc and area of sector! How do you do this please?

Use the formula for determining arc length.

s=r*theta where s is the arc length, r is the radius, and theta is the angle measured in radians.

Solve for theta

8.4 = 6*theta

theta = 8.4/6

theta = 1.4

If you want to convert 1.4 radians to degrees you do

1.4*180deg/pi = 80.2140913 degrees.