sigh this is killing me here...

Question

ok rockets launch at 250 m/s @ an angle of 75.0 degrees. There's a mountain 2500m away from launch point that is 1800m high. There's a boat 610 m from the island. So i'm suppose to find out how much it'll clear the moutain and close it'll come close to the boat

blah blah
so i find Vxi and Vyi
Vxi = 64.7 m/s
Vyi = 241 m/s

then I find time using formula
Vyf=Vyi+a delta t

solve for delta t and get 24.6s

try to find height use formula delta y = Vyi dleta T - .5g delta T^2 and I get 2960m

yada yada find distance traveled horizontally
d=rt
double time already found and got 49.2s
solve and got
3180m

so then I get three points
(0,0) (3180,2960) (3180,0)
I then use the lagrange method to find the equation of line

y = y1(x-x2)(x-x3)/[(x1-x2)(x1-x3)] +
y2(x-x1)(x-x3)/[(x2-x1)(x2-x3)]+
y3(x-x1)(x-x2)/[(x3-x1)(x3-x2)]

solve and I got -.001X^2 +3.72X
yada yada plug into graphic calculator and calc max and get vetex that is way off. The vertex should be the second point. i divided 3180 by two to get midpoint on horizontal axis and max height i already calculated... so (1590,2960) should be vertex but i don't get that
sigh I request help please tell me what i did wrong