Question: I have two points store in two variable, which forms a line. I want to find a point which is perpendicular to that line from one end point in that line.

Suppose I have two points P1(x1,y1) and P2(x2,y2) then i want to find a third point P3 such that line(P1-P2) is perpendicular to line(P2,P3) and intersect at P2.

Answer: 
If you want to use Java I can recommend to use [JTS](https://github.com/locationtech/jts). Create a [LineSegment](https://locationtech.github.io/jts/javadoc/org/locationtech/jts/geom/LineSegment.html) and use the pointAlongOffset method. Given Points p1 and p2 the code would look like that:

```
// create LineSegment
LineSegment ls = new LineSegment(p1.getX(), p1.getY(), p2.getX(), p2.getY());
// perpendicular distance to line
double offsetDistance = 10;
// calculate Point right to start point
Coordinate startRight = ls.pointAlongOffset(0, offsetDistance);
// calculate Point left to start point
Coordinate startLeft = ls.pointAlongOffset(0, -offsetDistance);
// calculate Point right to end point
Coordinate endRight = ls.pointAlongOffset(1, offsetDistance);
// calculate Point left to end point
Coordinate endLeft = ls.pointAlongOffset(1, -offsetDistance);

```
I got the answer at <http://jsfiddle.net/eLxcB/2/> 

```
// Start and end point
var startX = 120
var startY = 150
var endX = 180
var endY = 130
R.circle(startX,startY,2);

// Calculate how far above or below the control point should be
var centrePointX = startX
var centrePointY = startY;

// Calculate slopes and Y intersects
var lineSlope = (endY - startY) / (endX - startX);
var perpendicularSlope = -1 / lineSlope;
var yIntersect = centrePointY - (centrePointX * perpendicularSlope);

// Draw a line between the two original points
R.path('M '+startX+' '+startY+', L '+endX+' '+endY);

// Plot some test points to show the perpendicular line has been found
R.circle(100, (perpendicularSlope * 100) + yIntersect, 2);

```