solve runge kutta for eqution of one variable

lamda=0.2;
mu=0.8;
h=0.5;                                             % step size
x = 0:h:100;                                         % Calculates upto y(3)
Y = zeros(1,length(x)); 
%y(1) = [-0.5;0.3;0.2];
y(1) = -0.5; % redo with other choices here. 
    % initial condition
F_xy = @(t,%%) (lamda^2/(lamda+mu)^2)*exp(-2*(lamda+mu)*t)+((2*mu*lamda)/(lamda+mu)^2)*exp(-(mu+lamda)*t)+mu^2/(lamda+mu)^2;                  % change the function as you desire
for i=1:(length(x)-1)                              % calculation loop
    k_1 = F_xy(x(i),y(i));
    k_2 = F_xy(x(i)+0.5*h,y(i)+0.5*h*k_1);
    k_3 = F_xy((x(i)+0.5*h),(y(i)+0.5*h*k_2));
    k_4 = F_xy((x(i)+h),(y(i)+k_3*h));
    y(i+1) = y(i) + (1/6)*(k_1+2*k_2+2*k_3+k_4)*h;  % main equation
  
end
% validate using a decent ODE integrator  
tspan = [0,100]; y0 = -0.5;
[tx, yx] = ode45(F_xy, tspan, y0)
plot(x,y,'o-', tx, yx, '--')

how can i correct the function to run the code if i have this equation ?

[rohmatul]

F_xy = @(t,r)