Distributed load along a beam, Finite difference method and ode 45 errors

I have been working on this for a while but still not managed to get any graphs that make any sense, please see problem statement and code and graphs i have managed so far below

close all

clear all

clc

%variables

E=29*10^6; % elastic modulus (psi)

I=723; % moment of inertia (in^4)

q=500; %disrubuted load (lb/in)

L=36; %length

%equation

x=linspace(0,L);

y=((-q.*x.^2)/(24*E*I).*(6.*(L.^2)-4.*L.*x+x.^2));

%part a

%finite difference method

dx=6;

xx=0:dx:L;

yo=0;

yn=0;

ya=((dx.^2*q/(E*I))*((-L.^2/2)+L.*0-0.^2/2))/2;

A = [1 0 0 0 0;

-2 1 0 0 0;

1 -2 1 0 0;

0 1 -2 1 0;

0 0 1 -2 1];

z2=(dx.^2*q/(E*I))*((-L.^2/2)+L);

b = [(z2*xx(1)-xx(1).^2/2)+2*ya;

(z2*xx(2)-xx(2).^2/2)-ya;

(z2*xx(3)-xx(3).^2/2);

(z2*xx(4)-xx(4).^2/2);

(z2*xx(5)-xx(5).^2/2)];

yi=A\b;

yb=[yo yi(1:4)' yn yi(5)];

plot(x,y, 'r-',xx,yb, 'b')

hold

on

grid on

title('Graph of beam deflection')

xlabel(

'distance along beam(in)')

ylabel(

'deflection of beam (in)')

%part c

tspan=[0,36];

za=fzero(@res_project2,6);

y0=[0 za];

[x,yy]=ode45(@sys_project2,tspan,y0);

plot(x,yy(:,1),'k',x(1),yy(1,1),'og',x(end),y(end,1),'og')

legend(

'analytical','finite differance','shooting method','endpoints')

hold

off

%part d

xd=0:dx:L;

y=((((-q*x.^2)/24))/E*I);

ea=abs(y-ya);

figure(2)

plot(x,ea)

hold on

grid on

xlabel('distance along beam')

ylabel(

'abs error')

x=0:L:57;

[x,yy]=ode45(@sys_project2,tspan,y0);

ec=abs(y-yy(:,1));

plot(x,ec(:,1))

legend('fdm error','shooting method error')

% part e

% part f

% part g