Having trouble

[Jessica.c]

Ok so the question is: If a rock falls from a height of 20 meters on the planet Jupiter, its height H (in meters) after x seconds is approximatly H(x)=20-13x^2.

a) When is the height of the rock 15 meters?
b) When does the rock strike the ground?

I don't know how I wpuld start these problems off or what to do can someone please help explain to me how to do this. I have no problem with someone giving me an example of how I could figure this out.

Thank you,

Jessica.c

 

[wjm11]

If a rock falls from a height of 20 meters on the planet Jupiter, its height H (in meters) after x seconds is approximatly H(x)=20-13x^2.

a) When is the height of the rock 15 meters?
b) When does the rock strike the ground?

Don't make this too complicated; it's pretty straightforward. You are given an equation in which height is a function of time. For both problems, you just plug in the height and solve for time (x). So for the first one, it's

H(x)= 20-13x^2

15 = 20-13x^2

For part b, what is the height at ground level? It's zero, obviously. So,

0 = 20-13x^2

Just solve for x in both cases.

 

[Jessica.c]

Thank you very much. I appreciate it!