It can be solved using ode45 like this.
create 1 function file with the following instructions.
function dydt = swingeqn(~,del,pmax,M,D)
dydt=zeros(2,1);
dydt(1)=del(2);
dydt(2)=-(pmax/M)*sin(del(1))-D*del(2);
Now call it from script file
clc
clear all
pmax=-5;
M=1;
D=0.1;
tspan=[0,10];
del0=[0 1];
[t,del]= ode45(@(t,del) swingeqn(t,del,pmax,M,D),tspan,del0); % Calling the function swingeqn
plot(t,del(:,2));
xlabel('Time in Sec');
ylabel('Delta, Omega');
