Answer:
The rocket will reach its highest point about 2.04 seconds.
Step-by-step explanation:
The function:

Models the relationship between the height of the rocket and the time after launch t in seconds.
Since this is a quadratic function, the rocket will reach its highest point at its vertex. The vertex of a quadratic is given by:

In this case, a = -4.9, b = 20, and c = 0. Thus:

The rocket reaches its maximum height after 10/4.9 or about 2.04 seconds.
Further Notes:
Then the maximum height of the rocket will be f(10/4.9):

I believe it's 15, but correct me if I'm wrong
This is because there is one 10 in 15. This gives you 3 tens and 5 in the ones.
You did a simple mistake by forgetting to add 4 to itself in the square root. (-4^2=16)
x=(4 +/- sqrt(16-8))/4
x=(4 +/- sqrt(8))/4
Is this enough to get you started again?