a projectile is shot upwards with an initial velocity of $\displaystyle 30m/s$ , its height at time t is given by $\displaystyle h=30t-4.9t^2$, during what period of time is the projectile more then $\displaystyle 40m$ above the ground? express your answer rounded to the nearest hundredths

my thinking: $\displaystyle 40=30t-4.9t^2$ but can't go anywhere from there