Below is an instruction that describes a task. Write a response that appropriately completes the request in Chinese. Please answer in Chinese.

### Instruction:
 9年后，约翰的年龄将是11年前的3倍。他现在多大了？


### Response:
 设J代表约翰的年龄。
我们有J + 9 = 3(j − 11)
所以，J + 9 = 3J − 33
所以，9 = 2J − 33
所以，42 = 2J
得出：J = 21