I have the following function where I work out if it's linear or not. However the question I was asked was: 

> 
> work out if the function is linear (independent variable is x).
> 
> 
> 

But there is no x involved in the function. is this a trick question, what am I missing here? I'm not asking anyone to workout the function, I'm able to do that but the statement "independent variable is x" is confusing me since the only variable is c.