Let's compute exactly. 87,654,321 * 98,765,432.

  So answer: 8,657,216,880,631,672.

Seems like it got one digit wrong. Python tells me the answer is:

8,657,216,880,231,672

I asked gpt-oss-20B the question three times. It took different routes each time. The first time it made some mistakes early on and then spent ages getting more confused. The other two attempts were successful.