1. Note/Show - Inverse of every type of row operation is also a row operation
2. Use induction over number of row operations,n, needed for P->Q. Prove for the base case where n=1. You have to show that inverse of all three types of row operations restore the matrix to the original form.
3. Once this is done just use the induction logic to complete the prove.
I would assume this is a std way and can be found in most of the textbooks.