The greatest common factor is the factor that is the largest present among all of your numbers. So:
4 has factors: 1, 2, 4
20 has factors: 1, 2, 4, 5, 10, 20
24 has factors: 1, 2, 3, 4, 6, 8, 12, 24
Which number is largest among all of them? 4 is! So you factor our the 4 from your expression. When you do you, you are essentially dividing out each term by 4 in the parentheses:
\[4n^2-20n+24=4(n^2-5n+6)\]