Urgent Help Quadratic Fuctions/Relations

Heres the problem

Your a US general serving in Iraq and have been given orders to bomb a target 20km away. The missile launcher has been programed with an equation showing the parabola trajectory of the missile. If the equation is given in the general form y=ax2+bx+c how can you know if it will hit the target.

Give written instructions for how you would use the equation to answer the question. Give an example of an equation that would hit the target.

For the question i have to write written instructions on how to use the equation to see if it will hit the target.

For my work all I know is that I have to put 20 in the equation

y=ax2+20x+c and I think that ax2 and c are some variables like the wind and speed but I am not sure

Can anyone help me on this I only have less than 8 hours