Answer:

Make a withdrawal of $21
Explanation:
Given

(2 checks)


Solving (a): Show final balance
The transaction can be expressed as:

i.e.
Add cash inflow
Subtract outflow
This gives:





Solving (b): Get account balance to 0
To get his account to 0, he needs to make a withdrawal of the exact amount in his account (i.e. $21)
So, all he needs to do is to make a withdrawal of $21