NASA creates a weightless environment by flying a plane in a series of parabolic paths. The height h (in feet) of a plane after t seconds in a parabolic flight path can be modeled by h=−11t2+700t+21,000. The passengers experience a weightless environment when the height of the plane is greater than or equal to 30,800 feet. For approximately how many seconds do passengers experience weightlessness on such a flight? Round your answer to the nearest second.

The height is 30,800 feet or greater from about t=__ seconds to about t= __ seconds.

Please help, I have a test coming up and I don't understand these kinds of word problems!