Unfortunately it is kind of difficult to help with so little information. Could you paste a way to reproduce your arrays? Maybe uploading a smaller version of them and/or how to create them. It will be easier to debug then.
For example, in your code you don’t show the definition of XTXS, which you are using then when you do
MME= XTXS + LamI
b = RHS/MME
You can take a look at this thread with advice on what kind of info would make it easier.
PSA: make it easier to help you