Aball is thrown straight upward from ground level with a velocity of 18 m/s. how much time passes before the ball strikes the ground? disregard air resistance.

You need to know who high it went then from there you can go on. if it went up 72m then it would of took 8 second. four going up and four coming down

The initial speed will be slower to 0 and it will fall back to ground. The negative acceleration because of the earth gravity is 9,8 m/s2. 18 / 9,8 = 1,8367 second up, that point the speed will be 0.
The distance done during this time 16,531m and the ball will do this the same time back, while speeding up to 18m/s again. So I think it will take 3,673 sec to hit the ground.