a=0
b=2.5
dt=0.5

t=a:dt:b
y(1)=3
f=@(t,y)-1.2*y+exp(-0.3*t)
for i=2:length(t)
    y0 = y(i-1)+f(t(i-1),y(i-1))
    y(i)=y(i-1)+(f(t(i-1),y(i-1))+ f(t(i),y0))*dt/2
end
p=plot(t,y)
set(p,'Color','magenta')

y2 = 70/9*exp(-0.3*t)-43/9*exp(-1.2*t)
err = y2-y