You need only x7 and x8 initial
function main
[t,x] = ode45(@func,tspan,[x7 x8]);
plot(t,x)
function dx = func(~,x)
x1 = x(2)^2 + 3*x(1);
x2 = x(1) - x(2);
% x3 .. x6
dx(1,1) = -x5;
dx(2,1) = -x6;
end
end
