Answer:
c = 4
Explanation:
The number of solutions for any equation is equal to the degree of the equation.
The given equation is a second degree equation which means that it has two solutions.
The only possible way for it to have one unique real solution is that the two solutions are equal
This means that:
x - 4 = x - c
Now, we solve for c:
x - 4 - x = x - c - x
-4 = -c
c = 4
Hope this helps :)
