For what values of x is the function f(x)=x^2-4x-5 increasing?

For x ∈ [ - b / ( 2a ) ; +oo ) , f is increasing; => x ∈ [ +2 ; +oo ).

Slope of AB =  1/3 and slope of  BC = -3    so these 2 lines are perpendicular
The same is true for all the other adjjacent pairs of lines.
Oppoitse lines are also paralllel (  slope of AB = 1/3 and slope of  DC = 1/3)  and other pair are both of slope -3.

So Its C