Concavity question derivative

I'm very confused.

1) To find is the concavity is increasing or decreasing, do I substitute the zeroes of the first derivative into the first derivative or into the second derivative?

2)What is the purpose of substituting the zeroes of the first derivative into the original f(x)? Is it to find the location of the local maximum and minimum?

1) Into the second derivative (obviously not into the first derivative because that answer will always be 0).

2) Yes - to find the y-coordinates of the local maximum or minimum.