When the ball hits the ground, height = y will be ZERO
0 = - 16t^2 - 80t + 54 Use quadratic equation a = -16 b = -80 and c = 54 to solve for t
throe out the negative 't' value
\(t = {80 \pm \sqrt{(-80)^2-4(-16)(54)} \over 2(-16)}\)
You need to know how to calculate using the Quadratic Formula......if you already do, then here is a quick and easy Quadratic Formula calculator that I often use :
https://www.calculatorsoup.com/calculators/algebra/quadratic-formula-calculator.php