Help with projectile trajectory problem

Hi, im new using this software and i really need some help to make my code run. The teacher ask me to resolve this problem using matlab, but i dont really know if im doing well. It's about a projectile trajectory, i need to get the angle theta to get de code run.

This are the initial conditions:

m=15 [kg] g=-9.81 [m/s^2] v0=100 [m/s]

When the projectile touch the ground: xmax=300 [m] ymax=-50 [m]

Also he give me the eqn: my''=F

clear all;clc;close all

%%Constantes

syms theta
m=15; % [kg]
v0=100; % [m/s]
g=9.81; % [m/s^2]
x0=0; % [m]
y0=0; % [m]
ymax=-50; %[m]
xmax=300; % [m]
t=0; % [s]

%%Formulas

v0x=v0*cos(theta);
v0y=v0*sin(theta);
t=(xmax/(v0*cos(theta)));
x=v0x*t + x0;
y=(v0y*t-0.5*g*(t^2)+y0);
eq=xmax*tan(theta)-0.5*g*(xmax/(v0*cos(theta)))^2==ymax;

%%Para Theta

Angulo=solve(eq,theta);
vpa(Angulo);
for i=1:4;
    Angul=0;
    if ans(i)>0;     
       Angul(i)=ans(i)*180/pi;
       Angul(i)=double(Angul(i));

         if Angul(i)>0 && Angul(i)<90
             Respuesta=Angul(i);
             fprintf('El angulo del disparo del cañon es de %f\n',Angul(i));
         end
      end
  end

%%Ploteo

hold on

for theta=Respuesta;
    t=(xmax/(v0*cos(theta)));
    ezplot(x,y,'b');
end

I think im not doing well the code, but ill wait for awnsers.