6x2 + 1 – [8x – {3x2 – 7 – (4x2 – 2x + 5x + 9)}]
= 6x2 + 1 – [8x – {3x2 – 7 – 4x2 – 2x + 5x + 9}]
= 6x2 + 1 – [8x – 3x2 + 7 + 4x2 – 2x + 5x + 9]
= 6x2 – 1 – [8x + 3x2 – 7 – 4x2 + 2x – 5x – 9]
= 6x2 + 3x2 – 4x2 – 8x + 2x – 5x – 1 – 7 – 9
= x2(6 + 3 – 4) + x(8 + 2 – 5) – 15
= 5x2 – 11x – 15
Degree of the expression is 2.