If you're working with complex numbers, then I'm sure you're comfortable with plotting them on a complex-plane ... real part of the number along the x-axis, and imaginary part of the number along the y-axis.
When you look at it that way, your two points are simply two points on the x-y plane:
4 - i ===> (4, -1)
-2 + 3i ===> (-2, 3) .
The distance between them is
D = √ (difference in 'x')² + (difference in 'y')²
= √ (6)² + (4)²
= √ (36 + 16)
= √ (52)
= 7.211 (rounded)
Thirty four million one hundred twenty three thousand six
The answer is

.
Explanation:
In order to subtract the fractions, we must make them like fractions. To do this, the denominators must be the same by multiplying (only). Since the first fraction is

, 3 can be multiplied by 2 to get 6, which is the other fraction, we can multiply it. Whatever you do to the denominator you must do to the numerator. Now multiply 2 by the numerator (10) to get

. Now we can subtract the fractions

and

to get 13/6. Since this fraction is not in mixed fraction form yet, we must do that first. goes into 13 twice, so the whole number is 2 and there is still 1 left, making the fraction

. Therefore, the difference is 2

.
It's a factor. This concept is widely used throughout algebra, and you'll probably bump into it through the end of high school and beyond.
A common use is expressing a term in <em>prime factorization</em>, or reducing a number to its most base parts- primes. For example:

Of course, a number like 13 which is already prime is made up of itself and 1. <em>Factors do not have to be primes.</em> 20 is also reducible through combinations of 1, 2, 4, 5, 10, and 20. Prime factorization is just a handy example.
Basically, factors multiply with each other to create other numbers, and numbers can be reduced down to their factors.