Help with Line of Best Fit equation!

Question

Alright, I'm making a graphing program where the user can input up to 10 coordinates (all ints) and when they click draw, the points will be graphed on a cartesian plain and the program will calculate and draw the line of best fit... including displaying the equation.
I've got the yIntercept to work perfectly, but sometimes the slope comes out to be 0.0, and such.
The equation to do it can be found on this site (in the big white space)
http://people.hofstra.edu/faculty/Stefan_Waner/RealWorld/calctopic1/regression.html
There are several things you must do first:
- Find the sum of all the X values
- Find the sum of all the Y values
- Find the sum of all the X*Y values
- Find the sum of all the X^2 values
You then stick it into the equation (you can see on that site) and you will get the slope. You then take the slope value and insert it into another equation to get the y-intercept
I was just wondering if you could just look over my calculation and see where I may be going wrong. (Sometimes, I get an error in the console saying I cannot divide by 0)
public void lineOfBestFit(int numOfPoints, int largeNumX, float increment, float incrementY)
     Graphics g = canvas.getGraphics();
     float sumX = 0;
     float sumY = 0;
     float sumXY = 0;
     float sumXSqr = 0;
     int smallNumX=9999999;
     int firstPoint, lastPoint;
     //Creates the variables for the line of best fit equation
     for(int j=0;j<numOfPoints;j++)
          sumX+=xValue[j];
          sumY+=yValue[j];
          sumXY+=(xValue[j]*yValue[j]);
          sumXSqr+=(Math.pow(xValue[j],2));
     //Line of best fit equation
     float slope = ((numOfPoints*sumXY)-(sumX*sumY))/((numOfPoints*sumXSqr)-(sumX*sumX));
     float yInt = (sumY - (slope*sumX))/numOfPoints;
     for(int j=0;j<numOfPoints;j++)
          if(xValue[j]<smallNumX)
               smallNumX=xValue[j];
     firstPoint=((int)((slope*smallNumX)+yInt)*50/(int)incrementY+210);
     lastPoint=((int)(slope*largeNumX+yInt)*50/(int)incrementY+210);
     int firstX=((int)(smallNumX*50/(int)increment+210));
     int lastX=((int)(largeNumX*50/(int)increment+210));
     g.drawLine(firstX,firstPoint,lastX,lastPoint);
     //Rounds and displays the line of best fit equation
     slope = (Math.round(slope*100))/100;
     yInt = (Math.round(yInt*100))/100;
     if(numOfPoints>1)
                 if(yInt < 0)
          g.drawString("y = " +slope+ "x " +yInt,250,350);
          else
               g.drawString("y = " +slope+ "x +" +yInt,250,350);

Answer 1

I think I may have solved it.... seems to be working now.
I casted all the numOfPoints as (float) .... and commented out the rounding. Since some of the slopes were coming out to be 0.00003, that would display 0.0 if I rounded.