A = 5 x ² + 4 x - 1
B = - x ² - 3 x + 3
C = - 6 x ² - 7 x + 5
∴ A - B + C = 5 x ² + 4 x - 1 + x ² + 3 x - 3 - 6 x ² - 7 x + 5
= （5 + 1 - 6）x ² + （4 + 3 - 7）x - 1 - 3 + 5
= 0 x ² + 0 x + 1
= 0 + 0 + 1 = 1