OK don't panic poopy!
First of all, does the question really ask you to name the element? I know that that was the heading of your post but I'm wondering because of the other stuff you've posted since.
In your initial post, you didn't say which scale the 708 degrees was measured in. In your last post, you say it was 708 degrees C. Which is it?
If it does say 708 degrees C, then everything points to this question being worded to test your knowledge of the Periodic Table. This is because as you've rightly sussed, there isn't an element with an mp of 708 degrees C. The fact that it's referred to as "Element Z" and it asks which group of the Periodic Table it is "probably in" seems to support this.
Leave the mp to one side for a moment and look at the three other clues you've quoted in your first post. Then think of the oxides of the group you suspect may be possibilities and the solubilities of those oxides in HCl. It may help if you write down those other three clues on a separate sheet of paper and eliminate groups systematically when they don't match your criteria.
Incidentally, as you probably know, the most commonly used temperature scales nowadays are Celsius, Fahrenheit and Kelvin. At a push you could add in the Reaumur scale of historical significance and the Rankine scale used sometimes by american engineers.
It's many years since I carried around element melting points in my head so I checked this 708 figure out on the Knovel database via my university. However, if you use the two following websites together, you'll see that the 708 figure doesn't match any element.
http://www.science.co.il/PTelements.asp?s=MP
http://www.easysurf.cc/cnver14.htm#rec4