Need help fixing the "else" without "if" error when calculating positive or negative y-intercepts for the equation of a line

I'm making a program that writes the equation of a line for the user, given 2 x and y-intercepts. The "else" without "if" error is happening on line 46 when I try to calculate the y-intercept for a negative number. I'm pretty new to coding, so any help or suggestions would be greatly appreciated.

I tried to use just if statements, but then the issue is that the variable b is already declared, and I've also fiddled with squiggly brackets but those haven't worked for me too.

    import java.io.*;
    //Calculates the y=mx+b for user.

    class YMXB
    { 
    public static void main(String args[]) throws Exception
    { 

    BufferedReader br= new BufferedReader(new InputStreamReader(System.in));

    System.out.println("This program calculates the equation of a line (y=mx+b) for you.");

    //Input for coordinates
    double x1;
    System.out.print("Please enter x coordinate #1: ");
    x1=Double.parseDouble(br.readLine());

    double y1;
    System.out.print("Please enter y coordinate #1: ");
    y1=Double.parseDouble(br.readLine());

    double x2;
    System.out.print("Please enter x coordinate #2: ");
    x2=Double.parseDouble(br.readLine());

    double y2;
    System.out.print("Please enter y coordinate #2: ");
    y2=Double.parseDouble(br.readLine());


    //calculating slope
    double m=(y2)-(y1)/(x2)-(x1);

    //calculating y-intercept
    double x3=(m)*(x1);

    //if statements for +x3
    if (x3<0)
        x3=Math.abs(x3);
        double b=(y1)+(x3); 


    //if statements for -x3
    else if(x3>0)
        x3*=(-1);
        double b=(y1)-(x3);


    //calculating x and y
    double y= (y2)-(y1);

    double x= (x2)-(x1);


    //if statements for horizontal lines
    if (y==0) {
    System.out.print("This is a horizontal line.");
    System.out.print("y= "+x);
    }

    //if statements for horizontal lines
    if (x==0) {
    System.out.print("This is a vertical line.");
    System.out.print("y= "+y);
    }


    //y-intercept greater than 0
    if (b>0){
        System.out.println("The equation of your line is y= "+m+"x+"+b);
    }

    //y-intercept less than 0
    if (b<0){
        System.out.println("The equation of your line is y= "+m+"x"+b);
    }

}

}

When the user inputs the numbers the program should give the statement y=mx+b or y=mx-b, depending on if the y-intercept is positive or negative.