Can someone please help me? (1 Viewer)

[Epiphany]

I'm really stuck on this question and you all seem so good at maths I was hoping someone might know!
here it is:

Find the value of c such that y=x+c is a tangent to a parabola y=x-x-12. (hint: Consider the discriminant of the quadratic which results).

My working so far..
y=x+c
y=x-x-12

x-x-12=x+c
x-2x-12-c=0

b- 4ac
(-2)-4(1)(-12-c) = 0

now i'm not sure whats wrong, but any way that i try to solve it doesn't get me the right answer of -13.
I'd appreciate it so much.. sorry if it sounds like a stupid question.

 

[abdooooo!!!]

from: (-2)-4(1)(-12-c) = 0

4 + 4(12 + c) = 0
13(4) + 4c = 0

thus, c = -13

 

[Epiphany]

im not too sure of this line:

13(4) + 4c = 0

how did you get that?

 

[Epiphany]

ah never mind i got it! thank you so much

 

[abdooooo!!!]

4 = 4*(1)

4*(12 + c) = 12*(4) + 4*c

4 + 4*(12 + c) = 1*(4) + 12*(4) + 4*c [since: 1 + 12 = 13]

= 13*(4) + 4*c = 0