How to use I2C Pin as normal GPIO?

I have a VIM2 with ubuntu 16, and I want to use I2C_SCK_A and I2C_SDA_A as normal GPIO, error info raised when I export I2C_SDA_A, pin number 84

# echo 84 > export
-bash: echo: write error: Device or resource busy


what’s wrong?
could anyone give me a GPIO number map of the vim2?