If the radius of a circle is 3960

the angle is 1.59
the arc length is 110

how would I find the hypotenuse?

I think it is 3960/cos1.59
but I know that you just don't automatically know that wouldn't it start out as cos1.59 = A/H (adjacent/ Hypotenuse)

Could someone explain to me the beginning of the steps that result in 3960/cos1.59 ? I'm trying to find the hypotenuse.

I don't understand your question.

It looks like you have a sector of a circle, with radius 3960 and central angle 1.59°
Since arc = rØ, where Ø is the central angle in radians
arc = 3960(1.59π/180) = 109.89 which you gave as 110
If you draw the triangle in that sector wouldn't 3960 be the hypotenuse?
the x value would be 3960cos1.59° and the y value would be 3960sin1.59°