The mean is simply the average of the whole set; to find this, add up all the numbers (6450), then divide that number by the number of values in the set (there are 5 different sofa types in the set, to 6450/5, which is 1290. This is the average cost of a sofa in this set.)
The median is the middle-most value. However, this is not based on order within the list, but numerical value, so first set the values from least to greatest: 350, 1400, 1400, 1600, 1700. In this way we can see that the value in the middle, flanked on each side by two other values, is 1400, your median! For future reference, if you have two values in the center, just add them together and divide by 2.
The mode is the number which appears most often in the set. Because every number appears only once, except for 1400, then your mode is 1400. If you ever have a set with no recurring numbers, then it has no mode.
To reiterate: mean = 1290, median = 1400, and mode = 1400.
As for the last part of the question, what is the most misleading value to use in advertisement, you could argue that 1290 is the misleading number, as it indicates one could buy a cheap and already constructed sofa for less than 1300 dollars, while the 1400 dollar estimates are accurate and frequently found in the market. Hope this helped! If you have any follow up questions, feel free to ask.