Hi there!
Let's say you're talking about a rectangle here, of which the formula for is base * height = area. We can use this formula to figure out what the base of the polygon is. Let's plug in the values we know, using x in place of the base: x * 15 = 285. This gives us the equation: 15x = 285. Now, all we have to do to find the base is divide both sides by 15: x = 19 ft. The base of your shape is 19 feet.
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!