Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
−s6s67s59s52s48s49s39s310s212s212s15s1520
Step 2: Change ALL the signs in the second row.
−s6−s67s5−9s52s4−8s49s3−9s310s2−12s212s−15s15−20
Step 3: Combine like terms (add each column):
−s6−s62s67s5−9s516s52s4−8s410s49s3−9s318s310s2−12s222s212s−15s27s15−2035