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 stated before, finding the difference between two numbers, is just to subtract them. So if you want to find the difference, you take the bigger one minus the smaller one. 
But if you want to find the $distance$ between two number, you use the absolute value. For example, if you want to find the angle between two angles, lets say the angle between $90°$ and $30°$, we know it is $60°$ because the bigger one, $90°$, minus the smaller one, $30°$, is $90°-30° = 60°$. But what if you dont know which one is bigger? For example if you want to express the "difference" in the general case, using variables, you have to account for the fact that you don't know which is bigger, so you don't know if you should write $θ-ϕ$ or $ϕ-θ$. Then, to express that you want to find the difference between $ϕ$ and $θ$, you can write $|θ-ϕ|$, because when you take the absolute value of the whole expression, the order doesn't matter. And indeed, $|90°-30°| = 60° = |30°-90°| = |-60°| = 60°$.  In short, to find the difference, subtract the bigger one from the smaller one, to find $distance$ between the numbers on the number line, take the absolute value of one minus the other, because when you take absolute value of a subtraction, order doesn't matter.
