Answer
The function is
$$f(x)=-\frac{3}{50}x+27;$$
The domain is $(0,450)$;
The graph is on the figure below
Work Step by Step
Let $x$ denote the price in USD of Blu-ray players and $y$ average units sold per day. Then this linear function passes through points $P(200,15)$ and $Q(250,12)$. Then the equation of this linear function is given
$$y=\frac{12-15}{250-200}(x-200)+15=-\frac{3}{50}x+\frac{3}{50}\cdot 200+15=-\frac{3}{50}x+27,$$
so the required function is
$$f(x)=-\frac{3}{50}x+27.$$
Considering that the price should be positive and that the number of sold units should be positive as well, the domain is determined by $x>0$ and $f(x)>0\Rightarrow-\frac{3}{50}x+27>0\Rightarrow x<450.$ Finally the domain is
$$\mathcal{D}=(0,450).$$
The graph is the line passing through points $P$ and $Q$.
