This is asking you to subtract (a - 2b - 3c + d) from (a - 2b + 3c)
So it's written in order like this:
(a - 2b + 3c) - (a - 2b - 3c +d)
You need to distribute the negative (subtraction sign) to each term in the second set of parentheses.
This will change the negatives to a positives and the positives to negatives
a - 2b + 3c - a + 2b + 3c - d
The a's cancel, the b's cancel
You're left with
3c + 3c - d
which equals
6c -d
