How to get the tangent of a point on a line?

I’ve done a line in a coordinate system but I want to know the tangent of point. How can I get the tangent?

Edit your question (don’t answer it) to explain “I’ve done a line” by giving the formula you wrote. How’s a “tangent of a point” related to the line?

Is your question related to the math formula editor in LibO? Is it a numeric application in Calc? Is your question about mathematics? If yes, this Q&A site is dedicated only to LibO usage issues, not to classroom assignments.

The tangent to a point on a straight line is the same line. Perhaps you are looking, instead, for a line that is perpendicular to the line at that point? Unless the “line” you have constructed is a curve …

As @ajlittoz requests, what is the formula for the “line” you have constructed??