How to solve this FDTD Boundary Condition with matrix form?

Hi guys,

I need your helps about solving the problem below. I am trying to solve Stress Wave Propagation problem by using FDTD(Finite difference time domain) method. I have discretize the domain and now ı am trying to apply boundary conditions. I have the boundary condition equation below for i=1 and j=2,...,ns-1(AB boundary). The left hand side of the equation has known terms and the right hand side of the equation has unknown terms. I know that ı need to obtain the matrix form for the equation below. But there are too many unknown on the right hand side. I mean that there are more unknown than number of equations so ı can not solve it with lineer equation system solver like Ax = B form. (or ı dont know how to solve). What can ı do? What ı need to search to solve it? Thanks for answers...

This is my discretization;

I wan to apply the boundary condition below;

the equation for i=1 and j=2,...,ns-1 (AB Boundary Condition)

matlab code for the equation;

AB1 = -Pi-(lambda(i,j)+2*nu(i,j))*((4*u(i+1,j,k+1)-u(i+2,j,k+1))/(2*d_r));  
AB2 = (lambda(i,j)+2*nu(i,j))*(-3*u(i,j,k+1)/(2*d_r))+(lambda(i,j)/R(i,j))*(((v(i,j+1,k+1)-v(i,j-1,k+1))/(2*d_s))+u(i,j,k+1));
AB1 = AB2; % AB1 IS KNOWN,  AB2 IS UNKNOWN