6 + (-1) - 1 + 9 - 4 + 7

= 6 - 1 - 1 + 9 - 4 + 7 (Using the property of adding negative numbers)

= 4 + 9 - 4 + 7 (Simplifying the subtraction)

= 13 - 4 + 7 (Adding 4 + 9)

= 9 + 7 (Simplifying subtraction)

= 16 (Adding 9 + 7)

Therefore, 6 + (-1) - 1 + 9 - 4 + 7 = 16.