Answer:
Widgets should be sold by $38.88 to maximize the profit.
Step-by-step explanation:
Vertex of a quadratic function:
Suppose we have a quadratic function in the following format:

It's vertex is the point 
In which


Where

If a<0, the vertex is a maximum point, that is, the maximum value happens at
, and it's value is
.
In this question:
The profit is given by:

Which is a quadratic function with 
The maximum profit happens at the x of the vertex. Thus

Widgets should be sold by $38.88 to maximize the profit.