if you're familiar with the quadratic equation
[tex]\frac{ -b \pm \sqrt{b^2-4ac}}{2a}[/tex]
well, [tex]\frac{ -b}{2a} [/tex] is the x component of the vertex, in your case, the vertex is the highest point
figure out what t is at the highest point, and then substitute that back into the equation in order to determine height
x=t in this problem
