(m − n)2 − (m − 3n)2

Sure, here is the step by step solution:

1. Expand the squares in the expression:

   (m^2 - 2mn + n^2) - (m^2 - 6mn + 9n^2)

2. Simplify the expression by combining like terms:

   m^2 - 2mn + n^2 - m^2 + 6mn - 9n^2

3. Further simplify the expression:

   4mn - 8n^2

So, (m − n)^2 − (m − 3n)^2 simplifies to 4mn - 8n^2.