Determining if a function is linear based on independent variable x 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.
 A: It depends a bit on what kind of mathematical object $c$ is.
If $c$ is a number, we can evaluate $y$ and then $y$ is constant regarding the choice of $x$. Constant functions are linear in the sense of calculus, where a linear function is of the form $y(x) = m x + n$.
A: $3c-5$ is constant with respect to $x$. No matter what value you choose for $x$ the output $y=3c-5$. 
But this is still linear with respect to $x$. A function $f(x)$ is linear if its average rate of change
$$
\Delta(x_1,x_2)=\dfrac{f(x_2)-f(x_1)}{x_2-x_1}
$$
is constant for all choices of $x_1\ne x_2$. 
In your case define $f(x)=3c-5$ and observe for any $x_1\ne x_2$ you have
$$
\Delta(x_1,x_2)=\dfrac{f(x_2)-f(x_1)}{x_2-x_1}=\dfrac{(3c-5)-(3c-5)}{x_2-x_1}=\dfrac{0}{x_2-x_1}=0.
$$
A: It may be a trick question, but it likely is a typing error, since x and c are next to each other at least on usual english and german keyboards.
If this is a task that was assigned to you (eg. a homework), you may ask the person that gave you the task for it, but you could work out the solution for both variants of the task.
