Red aerial minifalres are used by some boaters in an emergency.The flight of one brand of flare,when fired at an angle of 70 degrees to the horizontal,is modelled by the function:
h= -9(t-3)^2+ 83
where h is the height (m) and t is the time (sec),since the flare was fired.
a) For how many seconds does the flare burn before it hits the water?
请写出演算过程.