How i can solve an initial value problem in a system of differential equations?

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Hi every one... Please i need to help me urgent... I have a system of 4 differential equations (first order) :

       dP/dx= f1(P,S,b+,b-)    P(0)=0;
       dS/dx= f2(P,S,b+,b-)    S(0)=0;
       db+/dx= f3(P,S,b+,b-)    b+(0)=0;
       db-/dx= f4(P,S,b+,b-)    b-(L)=0;

With x=(0->L). The problem is : i don't have the value of b- in x=0, but i have b-(L)=0. My question is : what's the method i can use in Matlab for solving the initial value problem in a system of differential equations?

thanks for help me