Answer:
9 seconds
Step-by-step explanation:
y = -4.9t² +42t +18.9
if the ball will be on the ground then y =0 and we need to find t by solving the quadratic equation
-4.9t² +42t +18.9 = 0
t= (-b±√b²-4ac)/2a, quadratic formula
t= {-42±√[42²-4*(-4.9)(18.9)] }/ 2(-4.9) , use calculator to solve
t= (-42 ±√2134.44)/ -9.8, solve the square root
t= (-42 ±46.2)/-9.8
one t = (-42 - 46.2)/-9.8 = - 88.2/ -9.8 = 9 seconds
another t = (-42 + 46.2)/-9.8 = 4.2 / -9.8 ≈ - 0.43 seconds reject this answer
