In a two week it was sunny 3/7 of the days. how many days were sunny?

There are 14 days in two weeks, so we can set it up like this:
3 = x
7   14
This is where you can do things a couple of different ways...
If you can see that one denominator is a multiple of another, like 14 is a multiple of 7, you can figure out how many times you have to multiply 7 to get 14, which is 2.  Multiply both the numerator and denominator of 3 over 7, and you end up with 6 over 14 (6/14), so there were 6 sunny days in 2 weeks.
Another way to solve for x in this type of equation is something I've heard called the "fish method" (called this because if you trace your path through the problem while working the steps, it makes a little fish-shaped doodle)
3 = x  
7   14
In this problem, you want to start with the 14 and multiply it by the number that is diagonal from it, which is 3.  3 times 14=42  Then you are going to divide that number, 42, by the number directly above or below it, which here the number below is 7.  42÷7=6, so x=6, and that means there were 6 sunny days in a two week period. 
I hope one of these ways helps you :)