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

### Instruction:
 元比他弟弟大7岁，也是他的两倍大。大卫多大了？


### Response:
 设大卫的年龄为x。元比他大7岁，所以Y年龄 + 7 = Y + 7 = x + 7
Y年龄 = x
Y = x - 7
元的年龄也是大卫的两倍，所以2 * x = Y年龄
2 * x = Y年龄
2x = Y年龄 + 7
2x = x + 7
2x = 7
x = <<7=7>>7