Hello, TH1!
You're 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 parabolic trajectory of the missile.
If the equation is given in the general form ,
how can you know if it will hit the target?
Code:

 ***
 * *
 * *

* *

  *            *  
(0,0) (20,0)
We know two points on the parabola: . and .
Plug them into the general equation: .
. . The equation (so far) is: .
. . The equation is: .
Note: Since we want a downopening parabola, .
If the general doesn't care how high the missile goes,
. . can be any negative quantity.