Graph of a function
A typical function can be expressed as
y = f(x)
which means we can give x any value and we obtain a value for y
It's said that y depends on x
The equation given in the question has the form
x = -3
Note there is no y in the equation, which means the value of y is unknown, only the value of x is certain.
In other words, when x = -3, y can have any value, and no other value of x can be used
The graph of the function is shown below
The graph is shown as a red line
for a relation to be a function, each value of x must have one and only one value of y
the relation is not a function because the only value of x has many values of y