Mike lives in Guangzhou. His friend Tom lives in Xi'an. This winter, Tom came to Guangzhou to see Mike. They were very happy to see each other. They asked a lot about each other's family and work. They talked about the weather in their cities, too. Guangzhou is quite warm in winter. It doesn't snow in winter in Guangzhou. Some people in Guangzhou have never seen snow. In Xi'an, it often snows in winter. Children like to play snowball fight and make snowmen. That is great fun. Mike hoped to visit Xi'an next winter.

（-6a³-6a²c ）÷(-2a²)等于____；