Step 1: Write second polynomial below the first one. Making sure like terms are lined up.
−20s7s7360s636s62700s5512s511200s43634s427480s313287s337440s222730s221600s12600s
Step 2: Change ALL the signs in the second row.
−20s7−s7360s6−36s62700s5−512s511200s4−3634s427480s3−13287s337440s2−22730s221600s−12600s
Step 3: Combine like terms (add each column):
−20s7−s721s7360s6−36s6396s62700s5−512s53212s511200s4−3634s414834s427480s3−13287s340767s337440s2−22730s260170s221600s−12600s34200s