(2x³-5x²+6x-11)+(6x³+4x²-6x+12)-(5x³+3x²-9x+25)

Write the first two polynomials one under the other
with like terms one under the other.  Then
combine term by term:

 2x³-5x²+6x-11
 6x³+4x²-6x+12
 8x³-1x²+0x+ 1

and then under that bottom line:

 8x³-1x²+0x+ 1

Write the third polynomial:

  8x³-1x²+0x+ 1
-(5x³+3x²-9x+25)

But to subtract we must change every sign inside
the parentheses.  Then we combine them:

  8x³-1x²+0x+ 1
 -5x³-3x²+9x-25
  3x³-4x²+9x-24

Edwin