The height of a ball is modeled by hx) = -16x^2+ 80x, where x is the number of seconds after the ball is hit. How long is the ball in the air in other words when will it hit the ground?

check the picture below.

well, the angle at the vertex d is clearly not a right-angle, let's check for the one at the vertex f which does look like,   however if it's indeed, that means ef is perpendicular to df, in which case their slopes will be negative reciprocal of each other.

so the negative reciprocal of the slope of ef is 1, however the slope of df is not 1, is 1/2, so nope, they're not perpendicular lines.

true

explanation: