Unknown problem with my code? (Beginner)

Hey, I'm new to this forum and just started Javascript because my maths teacher challenged me to create a program to solve factoring questions. My code stops working at around the 20th line, and I don't know why. I've been using jsfiddle.net as my code compiler if that makes a difference.

This is my code:

Code:

`var ax2 = prompt('What is the ___x^2?');`

var bx = prompt('What is the ___x?');

var c = prompt('What is the ___?');

//get the original question vars

/*-----\/BELOW IS NOT MY CODE!!\/-----*/

function gcf(a, b) {

if (b === 0) {

return a;

} else {

return gcf(b, a % b);

}

}

/*-----^^ABOVE IS NOT MY CODE!!^^-----*/

//euclid's algorithm (finds gcf)

var multiplier = gcf(gcf(ax2, bx), c);

//get the gcf for later use (multiplier(num + num)(num + num))

alert(multiplier);

ax2 = ax2 / multiplier;

bx = bx / multiplier;

c = c / mulitplier;

//divides question vars by gcf

alert('We have divided by multiplier');

var abx3 = ax2 * bx;

for (i = 1; i++; i <= Math.sqrt(abx3)) { //start for loop

if (abx3 % i === 0) { //if i goes evenly into abx3

if (abx3 % i + i == bx) { //if the two possible factors equal bx

var num1 = ax2;

var num2 = abx3 % i;

var num3 = ax2;

var num4 = i;

}

}

}

alert('We have finished for loop!');

var num12 = gcf(num1, num2);

var num34 = gcf(num3, num4);

alert('We have found GCF of the two numbers for each bracket!');

//You really are awesome for reading this far!!

num1 = num1 / num12;

num2 = num2 / num12;

num3 = num3 / num34;

num4 = num4 / num34;

alert('WE have divided each number by the respective gcf!');

alert(multiplier + "(" + num1 + "x + " + num2 + ")(" + num3 + "x + " + num4);

I use the alert() thing to let me know when the code has passed a certain point so I can pinpoint where the problem is. Unfortunately, I know where the problem is, I just don't know what it is.

Thanks for taking the time to read this and, if you decide to help out, thanks for helping out!

-Tim

*Sorry if this is in the wrong subforum, I didn't really understand what the different subforums were for, so I picked what I think is the most general one.*