It takes bob 3 hours less than Joe to rebuild an engine.Working together,they can do the job in 6 hrs. How long would it take each,working ALONE ,to rebuild the engine?

let x - joe

1/x + 1/(x-3) = 1/6

6/x + 6/(x-3) = 1

6x - 18 + 6x = x(x-3)

12x - 18 = x² - 3x

x² -15x + 18 = 0

a = 1 b = -15 c =18

x = 15±√225 - 72   / 2

x = 15 ± 12.37 /2 

x =  13.68 Joe

Bob = 13.68 - 3 = 10.68