I seriously need help understanding the concept
The formula for the length of an arc on a circle is:
Arc Length = θ*r
Where r is the radius of the circle and θ is the angle subtended by the arc, measured in radians. Note that for a full circle, θ = 2pi radians and the Arc Length = 2(pi)r, the formula for the perimeter of a full circle.
In your problem, we know the radius (r) is 20 cm and the angle is 7 degrees (3.5 degrees on either side of vertical). All you need to do is convert the 7 degrees to radians then use the Arc Length formula above. The conversion formula is:
7 degrees 180 degrees
----------- = ---------------
x radians (pi) radians
Solve for x, then plug it into the Arc Length equation along with the radius, r = 20 cm, to compute the arc length.