The diagram includes a parabola with the point (6, 1).
“The figure above shows the graph of the quadratic function f(x)=a(x-4)^2+b, where a and b are constants. Which of the following number lines represents the set of all values that satisfy the inequality f(x)=a(x-4)^2+b <= 1?"

The answer was between 2 and 6 inclusive, but not sure how to get to it? (May 2013 SAT)

Thanks!
Not sure if when i submitted it on the other q and a it went through so posting here incase it didn't.