Rewriting your equation gives you 
We know this graph is an upside down parabola, and in order to find the maximum, you need to find the vertex of the graph. If you have a graphing calculator you can do it on there, if not... here's how to do it.
The vertex is nothing more than a point on the graph, so we need to find the X and Y coordinates to find the ordered pair.
The formla for the X value is 
Then once you find your X value, plug it back into your equation to find R.
That ordered pair will be your vertex, and the x value will be the total units sold to maximize the revenue.
You do not have to do both parts of this problem, once you find x, that will be your answer, but if you have any more where you need to find your y value, you'll be able to do that the same way as I explained here.