I'm reading a proof concerning a factor ring R/I. And I have come across the following sequence of steps: 1. Since R/I is commutative then (r+I)(s+I)=(s+I)(r+I)  (I is an ideal of R) 2. This implies that rs+I=sr+I 3. This implies that rs-sr is an element of I I don't see how step 2 implies step 3

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