There are a number of ways to approach this. The simplest is to use the equation
where s is the final displacement <r, 0> (r is the range, an unknown), s_0 is the initial position <0, 0>, v_0 is the inital velocity, and a is the acceleration. This equation decouples into x and y equations. What you want is the y equation. This only has as a variable.
PS: This should be in the Math subforum.