Question

The function f(t) = -18t2 + 42t + 36 represents the height of the path traced by an airborne flying disc, where t is the time in seconds. How many seconds will it take the flying disc to complete its path?

Answer 1

To complete path means the disc lands on the ground, the height f(t)=0
-18t²+42t+36=0
simplify by dividing -6: 3t²-7t-6=0
facotor: (t-3)(3t+2)=0
t=3 or t=-2/3 (discard this negative one because time cannot be negative. 
so the answer is 3 seconds.