If a rock falls from a height of 38 meters on Earth, the height H (in meters) after x seconds is approximately H(x)=38-4.9x^2.
When is the height of the rock 11 meters? The answer is 2.35 seconds, but I don't see how they got that answer. Can someone please explain this to me?

Question

If a rock falls from a height of 38 meters on Earth, the height H (in meters) after x seconds is approximately H(x)=38-4.9x^2.
When is the height of the rock 11 meters? The answer is 2.35 seconds, but I don't see how they got that answer. Can someone please explain this to me?

stamp · Answer

$$H(x)=38-4.9x^2$$Find x for H(x)=11 m.$$11=38-4.9x^2,\ 27=4.9x^2$$$$x=\sqrt{\frac{27}{4.9}}=2.35s$$

anonymous · Answer

Thanks! :)