Expected value of a continuous random variable

Suppose you have the following distribution function:

$F(x)=\left\{\begin{matrix} 0, x>3 \\ a (x - 1), 1<x \leq 3 \\ 0, x \leq 1 \end{matrix}\right.$

Calculate the value of the parameter a. The answer should be a real number with the accuracy of one decimal place. For example, $0.9$.