Backward differentiation of a mass matrix, Using Fzero function

Hi, Following is my code. Initial conditions are given on the top of the code. I have specified an eqn which is given in inline function. A3 is a matrix of 49788*688 values and I want to find X for every values of A3. I am using fzero function but I think I am doing some mistakes in its syntax. Moreover, it say subscripts indices must be real or positive integers and there is some error in initial conditions. Can anybody correct this code? Q is equal to 49788

 Cp(0)=1;
  Cp(-1)=1;

for k                       = 1:Q;
    fx                      = inline('3.5 *(X - 2*Cp(k-1) + Cp(k-2)) + 106*(X-Cp(k-1)) * abs(X-Cp(k-1)) + X- A3','A3');
    [x,feval]               = fzero(fx,0,Cp(0),Cp(-1),A3);
    Cpi(k)                  = X;
end