I have no idea what you are asking for. Your question is very vague.

You could perhaps use a measuring device like a ruler for a short distance, or a measuring tape for longer ones.

In case it is a line on a cartesian plane, you can use Pythagoras' Theorem.

You could perhaps make use of the cosine rule if you have a triangle with an angle theta between the two other sides.

A^2 = B^2 + C^2 - 2ABcos(\theta)

Or the sine rule:

\frac{A}{sin(a)} = \frac{B}{sin(b)} = \frac{C}{sin(c)}

As I said before, your question is vague. :(