When completing the square on the equation c 2 11c = 12, the resulting equation is:

C^2 + 11c = 12

c^2 + 2[11/2] c = 12

[c + 11/2] ^2 - [11/2]^2 = 12

[c +11/2]^2 = 12 +[11/2]^2

[c +11/2]^2 = 12 + 121/4

[c +11/2]^2 =[48+121]/4

[c +11/2]^2 = 169/4  > This is the resulting equation

Solving it you get:

c + 11/2 = +/- √(169/4)

c + 11/2 = +/- 13/2

c = 13/2 - 11/2 =2/2 = 1 and

c = -13/2 - 11/2 = - 24/2 = -12