What do you specifically do not understand?
with explanation and reasons please question 3d
AE = 4, AB = 7. So EB = ...?
CD = l, ED = x, so CE = ...?
Substitute these values in equation (1) and simplify.
The equation obtained is
To get the minimum length of the chord CD, find dl/dx and equate it to zero. Then solve for x. Substitute this value in eq.(1) to find CF, then find CD.