Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
−x6x67x59x52x48x49x39x310x212x212x15x1520
Step 2: Change ALL the signs in the second row.
−x6−x67x5−9x52x4−8x49x3−9x310x2−12x212x−15x15−20
Step 3: Combine like terms (add each column):
−x6−x62x67x5−9x516x52x4−8x410x49x3−9x318x310x2−12x222x212x−15x27x15−2035