What is the objects final displacement

Let f(x) = x4- 8x2 . find the relativeextrema of this function using the second derivative test.

The place you get your hair cut has two nearly parallel mirrors 6.00 m apart. as you sit in the chair, your head is 2.00 m from the nearer mirror. looking toward this mirror, you first see your face and then, farther away, the back of your head. (the mirrors need to be slightly nonparallel for you to be able to see the back of your head, but you can treat them as parallel in this problem.) how far away does the back of your head appear to be?

At what time will the interfering waves have an amplitudeof zero?