Maths Projectile.

An Object is projected vertically upwards from ground level and it's height, H metres, after t seconds is given by the formula:

$H=60t-5t^2$

Show that the object is above a height of 90 metres for about 8.5 seconds
working:

$90=60t-5t^2$

$5t^2-60t+90=0$

$t^2-12t+18=0$

Then I used the quadratic formula and got 10.24

where have I gone wrong?

An Object is projected vertically upwards from ground level and it's height, H metres, after t seconds is given by the formula:

$H=60t-5t^2$

Show that the object is above a height of 90 metres for about 8.5 seconds
working:

$90=60t-5t^2$

$5t^2-60t+90=0$

$t^2-12t+18=0$

Then I used the quadratic formula and got 10.24

where have I gone wrong?