If you plot the Total Money vs the Number of Cameras Sold, you will get the following equation:
Money = -4*Camera2 + 960*Camera
When you differentiate dM/dC = -8*Camera + 960 (Set this to 0 to find the maximum)
Setting this = 0, 8*Camera = 960 & you find that the Number of Cameras for a maximum = 120 Cameras.
The highest total money for 120 Cameras is -4*(120)2 + 960*Cameras = $57,600