The time of takeoff of this hovercraft would be 0 seconds, as if you were starting a timer. So, if you plug in 0 for x (unit for seconds) in your equation, it will look like this:
h(0) = -3(0 - 3)² + 108
Parenthesis first
h(0) = -3(-3)² + 108
Exponents next
h(0) = -3(9) + 108
Multiplication
h(0) = -27 + 108
Combine like terms
h(0) = 81: The height of the hovercraft at the time of takeoff is 81 meters!
Hope this helps :)