AMO 2020 Secondary 1 (Grade 7) Question 13

Use each digit 1, 2, 3, 4, 5, 6, 7 once to fill the boxes in $M=\boxed{\phantom{000}}-(\boxed{\phantom{00}}-\boxed{\phantom{00}})$, forming one three-digit number and two two-digit numbers. Find the greatest possible value of $M$.