Given the probability density function
$$ f(x) = \begin{cases} \frac{cx}{3}, & 0 \leq x < 3, \\ c, & 3 \leq x \leq 4, \\ 0 & \text{ otherwise} \end{cases} $$
I have found $c$ to be $0.4$ and $E(X)$ to be $2.6$. But I'm being asked to find $E(3X - 5)$ and I'm unsure of what to do.
$$\mathbf E(3\mathbf X-5)=3\mathbf E(\mathbf X)-5=3(2.4)-5=2.2$$