Question 864482
{{{h = -5 t^2 + 15t + 100}}}
Set h = 10
{{{10 = -5 t^2 + 15t + 100}}}
add -10 to each side
{{{0 = -5 t^2 + 15t + 90}}}
multiply by -1
{{{5 t^2 - 15t - 90 = 0}}}
divide by 5
{{{t^2 - 3t - 18 = 0}}}
factor
{{{(t-6)*(t+3)= 0}}}
Possible values for t are 6 and -3
So the firecracker has been in the air 6 seconds.