To factor this expression, the first thing to do is to take out the terms and look at the factors of each one:
-5p:-1*5*p
20: 2*2*5
When you look at the factors for each term, notice that both the terms have the same greatest common factor of 5. Noticing this, factor out 5 from the expression which will get you to 5(-p+4). However, normally in math, the first term of an expression usually is not a negative so you would factor out -1 from the expression which would result in your answer: -5(p-4)