Hello!
The answer is:
Yes, the given equation is also a function. (it's a quadratic function)
Why?
We know a function exists when there is only one value of y (output) for each x (input).
We can easily know if an equation (relation) is also a function using the graphic method:
To know if a relation is a function, we need to graph it a draw a vertical line, if the line cut the graph in only one point, the relation is a function, otherwise, if the vertical line cuts the graph in two or more points, the relation is a not a function. Remember, the condition for a function is that there is only one output for each input.
Note: I have attached a picture for better understanding.
Have a nice day!