@Preben Alsholm thank you ! u've help me a lot!

@Carl Love ok i got it ! thanks for guiding me! i've finish my assgnment..thanks a lot!

@Preben Alsholm I've tried dsolve(ode) and it gives no output at all  . What's mean by "res"?? & how should we use that?? 

@Carl Love oh I see.. Ok thanks a lot!