`h(t)= -5t^2 + 50t` .

We need to find the time needed for the firecracker to reach a height of 45m.

Then we will subsitute with h(t)= 45

==> -5t^2 + 50t = 45

==> `-5t^2 + 50t - 45 =` 0

Now we will factor -5:

==> `-5(t^2- 10t +9) = ` 0

Now divide by -5.

==> `t^2- 10t+ 9 =` 0

Now we will solve the quadratic equation for t.

we will factor.

==> (t-1)(t-9) = 0

==> t1= 1

==> t2= 9

Then there are two answers.

The firecracker will be at height 45 m after **t1=1 second**,
and then again after **t2=9 seconds** when the firecracker is
falling toward the ground.

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