Answer
The given equation DOES NOT define a function.
Work Step by Step
Using a table of values, the graph of the given equation (red graph) is shown below.
The Vertical Line Test states that if a vertical line intersects the graph of the equation in at most $1$ point, then the equation defines a function.
Since in the graph below, the vertical line $d=-2.5$ intersects the graph of the given equation in more than one point, then the given equation DOES NOT define a function.