(15x-121×12+x9-x7+3×2)+(x7-68×2-x9)

Remove parentheses.

15x-121×12+x9-x7+3×2+x7-68×2-x9

Add -x7 and x7.

15x-121×12+x9+3×2+0-68×2-x9

Add 15x-121×12+x9+3×2 and 0.

15x-121×12+x9+3×2-68×2-x9

Subtract x9 from x9.

15x-121×12+3×2-68×2+0

Add 15x-121×12+3×2-68×2 and 0.

15x-121×12+3×2-68×2

15x-121×12+3×2-68×2

Subtract 68×2 from 3×2.

15x-121×12-65×2

Add (15x-121x^12+x^9-x^7+3x^2)+(x^7-68x^2-x^9)