Answer
a) $g(x)$ is linear. $f(x)$ isn't linear because it has a 2nd degree of a polynomial ($x^2)$.
b) See graph
c) $-5$
Work Step by Step
a) Linear functions have only 1 degree of a polynomial, meaning that its variable is $x^1$. $f(x)$ isn't linear since its maximum degree of a polynomial is 2 ($x^2$).
b) See graph
c) The rate of change of a linear function is also its slope which is the constant of the variable $x$. In this case, it is $-5$