Determine if Linear f(x)=13-x

The word linear is used for a straight line. A linear function is a function of a straight line, which means that the degree of a linear function must be ${\displaystyle 0}$ or ${\displaystyle 1}$. In this case, The degree of ${\displaystyle f\left(x\right)=13-x}$ is ${\displaystyle 1}$, which makes the function a linear function.