@vv thankyou so much, that can help me a lot, thank's again

@Henk 

okk, I will try this,thank you Henk

@Henk 

Thank you Henk,but in the exercice, I need to do it with the Finite Difference Method, so do you have any ideas about that

@Carl Love Thank you Carl for your comm, I did what you said but it's still the same,it's difficult to solve it,I'm just training to solve it with Finite Difference Method, but found this problem. So, please help me if y have any ideas about this method