Help with math problem

Jeff and Kirk can build a 75 ft wall together in 4 hours. Because Jeff has more experience, he could build the wall by himself 3 hours quicker than Kirk. How long would it take Kirk, (to the nearest minute) to build the wall by himself?

I know the answer is 9 hours and 46 minutes but I have no idea how they got it. Can someone help?

Suppose building the wall requires an amount of work W.
Assume Kirk can do it by himslef in x hours. Then Jeff can do it by himself in (x-3) hours.
In each hour, Kirk completes W/x amount of work, and Jeff completes
W/(x-3)
When they wok together, in one hour they will complete (W/x)+(W/(x-3)) amount of work which is say y=W*(2x-3)/[x(x-3)] using common denominator

So the number of hours they would take to finish is W/y
=[x(x-3)]/(2x-3) where W cancels out
But this is given to be 4

So [x(x-3]=4(2x-3)
x^2-3x=8x-12
x^2-11x+12=0
Using quadratic formula, the discriminant is 11^2-48=73
So we have
x= [11+sqrt(73)]/2 or [11-sqrt(73)]/2
The first value is approximately 9.77 and the second value is 1.23. The 1.23 value is impossible since then x-3 will be negative.

So Kirk will take 9.77 hours to complete the work by himself. 9.77 hours is approximately 9 hours 46 minutes