How dor i find whay the radius of the curve if the middle ordinate measures 4.8 feet when using a chord with a length of 42 feet?

Use the intersecting chord theorem:

When two chords cut each other into
p1,p2 and p3,p4, then
p1*p2=p3*p4.

In the given case
(42/2)(42/2)=4.8(2R-4.8)
Solve for R.