After 5 seconds the rock should be 425 feet in the air.Workings are in the attachments below. Please read carefully.
The expression would show three sevenths of y minus four.

1. x = 3, y = 2
2. x = -99/68, y = -14/17
3. x = -4, y = 1
4. x = 1, y = 5
5. x = -1, y = 0
6. x = 2, y = -3
7. x = 14, y = 20
I attached the pictures with the work.