The graph of the function f(x) = (x − 3)(x + 1) is shown.

which describes all of the values for which the graph is positive and decreasing?

a. all real values of x where x < −1
b. all real values of x where x < 1
c. all real values of x where 1 < x < 3
d. all real values of x where x > 3

Option A. All the real values of x where x < -1

Procedure
Solve the inequality:
(x -3)(x+1)>0
That happens in two cases.

1) When both factors >0
x-3>0 and x+1>0
x>3 and x >-1

The intersection is x >3

2) When both factors <0
x-3<0 and x+1<0
x<3 and x<-1

the intersection is x<-1.

We have obtained that the function is positive for the intervals  x < -1 and x > 3. But in one of those intervals the function is decresing and in the other is increasing.

You can recognize that the function given is a parabola and, because the coefficient of the quadratic term is positive, the parabola opens upward. Then the function is decreasing in the  first interval and increasing in the second interval.