How should we calculate difference between two numbers? if we are told to find the difference between 3 and 5,

then we usually subtract 3 from 5 ,5-3=2
  and thus, we say that the difference is 2.
              but why can't we subtract 5 from 3 ,3-5= -2 to get the
  difference -2??
  which result is right?
  is the difference ( 2) or (-2)?

Also tell me how can we calculate the difference if we are told to calculate difference between two numbers,-5 and 2 on the number line.

As I understand it, the difference between two given numbers is a result of subtraction, so it depends on the order of data:
the difference between $5$ and $3$ is $5-3 = 2$, while
the difference between $3$ and $5$ is $3-5 = -2$
However the distance on the number line is a modulus (an absolute value) of the difference, hence independent on the order:
$dist(3, 5) = |3-5| = |-2| = 2 = |2| = |5-3| = dist(5, 3).$
