A withdrawal from an account means taking money out of your account. That means whatever money you have in there becomes a smaller amount because you're subtracting money from the account.
So, to work out how much money Jan withdrew as a negative sum, you just have to look at how much she took out:
-25 - 45 - 75 = -$145
This means Jan took out $145 from her account, and we write it with a negative sign because it's money that's going to be taken away from whatever's in there.
The same thing applies to Julie:
-35 - 55 - 65 = -$155
Julie took out $155 from her account, so her withdrawals were -$155.
Therefore, your final answer for 24a. is -$145 and your answer for 24b. is -$155.