Question 1159348
Draw a parabola to show the path of the arrow.
The equation is height=-5t^2+40t, the -5 is acceleration of gravity (-4.9 more accurately) and the 40 is the speed of the arrow in m/sec. The units are (m/sec^2)(sec^2) or meters and (m/sec)*sec or meters.
t is the number of seconds
h(5)=-5(25)+40(5)=75 meters high

the arrow is 30 meters high twice, going up and coming down
30=-5t^2+40t
rewrite to make t^2 term positive. It's easier that way.

5t^2-40t+30=0
divide by 5 to simplify it
t^2-8t+6=0
quadratic formula
t=(1/2)(8+/8+/-sqrt(64-24) or (1/2)* 8 +/- sqrt (40) which is 2 sqrt(10)
t=4+/- sqrt (10) multiplying it by the 1/2
This is at t=7.16 seconds and 0.84 seconds. You can make those to more decimal places if you wish and show that they will both give 30 m when put into the equation.

it hits the ground when H(t)=0
-5t^2+40t=0
5t^2=40t
t^2=8t
t=8 seconds
If you substitute 8 into the equation, it equals 0.

{{{graph(300,300,-10,10,-20,100,-5x^2+40x,30)}}}