You can go HERE:  http://www.mathopenref.com/coordtrianglearea.html  and use the formula described there:

and here's my solution:

import java.util.Scanner;
 
public class Pr_02_TriangleArea {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
 
        String[] sideOne = scanner.nextLine().split(" ");
        String[] sideTwo = scanner.nextLine().split(" ");
        String[] sideThree = scanner.nextLine().split(" ");
 
        int aX = Integer.parseInt(sideOne[0]);
        int aY = Integer.parseInt(sideOne[1]);
 
        int bX = Integer.parseInt(sideTwo[0]);
        int bY = Integer.parseInt(sideTwo[1]);
 
        int cX = Integer.parseInt(sideThree[0]);
        int cY = Integer.parseInt(sideThree[1]);
 
        //CHECK IF CORRECT // System.out.printf("%d %d %d %d %d %d",aX,aY,bX,bY,cX,cY);
 
        //USING THE FORULA FROM: mathopenref.com/coordtrianglearea.html
        //WHICH IS: |((aX*(bY-cY) +bX*(cY-aY)+cX*(aY-bY)))/2|
 
        int areaTriangle = ((aX * (bY - cY) + bX * (cY - aY) + cX * (aY - bY))) / 2;
        System.out.println("The area of the triangle is: " + Math.abs(areaTriangle));
    }
}