So here's the question:

The cost of fuel to propel a boat through the water (in dollars per hour) is proportional to the cube of the speed. A certain ferry boat uses $100 worth of fuel per hour when cruising at 10 miles per hour. Apart from the fuel, the cost of running this ferry (labour, maintenance, and so on) is $675 per hour. At what speed should it travel so as to minimise the cost per mile traveled?

The graph should be somewhat like this i think,


but problem is I can only find the equation for the red line from the information given, how can I find the equation for the curve? In need to be enlightened.