This question is finding the critical value by set the first derivative equals to zero.May you explain/ walk me through each step the author has performed? I do not now how does he went from one way suddenly to another, thank you very much.(I do understand the derivative process)

39. F(x) = 24/5(x - 4)2 F' (a) = 24/5. 2(20 - 4) + (2-4)2. -1/5 = ac -1/5 ( 20 - 4 ) 15 . 2 . 2 + ( 20 - 4 ) . 4] (xx - 4) (14x - 16) 2(2 - 4) (7x - 8) 521/5 521 /5 F' (x) = 0 = 4, 3. F'(0) does not exist. Thus, the three critical numbers are 0, $, and 4