You simply have to make t the subject by using the " completing the square " method.

Hence, step by step,

=> s= -16t^2 + 16t +320

=> -s = 16t^2 - 16t - 320

=> -s = (4t)^2 - 2(8)t - 320

Note: (4t - 8)^2 = 16t^2 - 16t + 64 || compare -s+320: 16t^2 - 16t

the excess is 64, right? simply, minus 64.

=> -s = (4t - 8)^2 - 320 - 64

Now, to make t the subject,

=> -s + 64 + 320 = (4t - 8)^2

=> (4t - 8) = (-s + 64 + 320)^(1/2)

=> t = (1/4)[8 + (-s + 64 + 320)^(1/2)]

In the end, you simply have to sub the values, and you have t ( time taken for the ball to hit the ground ).

since s is defined as the height of the ball above the ground, then s = 0 when it hits ground.

=> t = (1/4)[ 8 + (0 +64 + 320 )^(1/2)] = 6.90 ( 3.s.f )

Hm, not really sure, but note that the question gives hint to what s.f it wants, and basing on the values they give 16 and 320, it could want the answer in 2.s.f; hence,

t = 6.898979 ... = 6.9 ( 2.s.f )

Hence, time taken for the ball to hit the ground is 6.9s.