Ok, so:
We're going to factor this expression:
3a² - 24a + 48
First of all, we multiply and divide by 3 all the expression, like this:
3(3a² - 24a + 48) / 3
Now, we can rewrite this to a new form:
( (3a)² - 24(3a) + 144) / 3
Then, we have to find two numbers, whose sum is equal to -24 and its multiplication is 144.
And also we distribute:
((3a - 12 ) ( 3a - 12 )) / 3
Notice that the numbers we're going to find should be inside the brackets.
So, these numbers are -12 and -12.
Now, we factor the number 3 in the expression:
(3(a-4)*3(a-4))/3
And we can cancel one "3".
Therefore, the factored form will be: 3 (a - 4) (a - 4)
So, the answer is B.