Gaussian Elimination

For an assignment i am doing at uni i have been asked to produce a spreadsheet that will solve a set of 5 simultaneous equations using gaussian elimination.

I have set up the spreadsheet to do this, however, we have also been asked to make it work if we get a zero on the leading diagonal. This means that the equations would have to be rearranged.

This is the part im not sure how to do. Obviously i would be starting with an 'IF' expression, but other than that i dont know where to start.