-4 because -16-4=-4. Negative and a positive make a negative
First, let's start off by simplifying as much as possible before factoring.
5d^2 - 50d + 125
We can divide all the numbers by 5 in order to simplify it:
5(d^2 - 10d + 25)
Now all we need to do is factor.
Since -5*-5 = 25 and -5 - 5 = -10, we can use this to factor:
5(d - 5)(d - 5)
Or
5(d - 5)^2
