Answer:
In line 4, he did the mistake. He should have square rooted the other side as changing sides changes square to square root.
explanation:
line 1: [tex]\sf p^2 - 10p - 11 = 0[/tex]
[tex]\sf p^2 - 10p = 11[/tex]
line 2: [tex]\sf p^2 - 10p + 25 = 11 + 25[/tex]
[tex]\sf p^2 - 5p -5p+ 25 = 36[/tex]
[tex]\sf p(p-5)-5(p-5) = 36[/tex]
line 3: [tex]\sf (p-5)^2 = 36[/tex]
line 4: [tex]\sf p - 5 = \pm \sqrt{36}[/tex]
[tex]\sf p-5 = 6[/tex] or [tex]\sf p-5 = -6[/tex]
line 5: [tex]\sf p = 11[/tex] or [tex]\sf p = -1[/tex]
line 6: The solution is p = 11 or p = -1
