If Stuart starts with -$54.19 then gains $70.82, you have to add the gained amount to the origin amount. Since the origin amount is negative, you can also subtract the origin amount from the gained amount to find the amount left at the end of the month.
Add 70.82 to -54.19
-54.19 + 70.82 = 16.63
Alternative method:
First, subtract 54.19 from 70.82.
70.82 - 54.19 = 16.63
Add 54.19 to -54.19, bringing you to 0.
-54.19 + 54.19 = 0
Now add the remaining 16.63.
0 + 16.63 = 16.63
Answer:
Stuart will have $16.63 at the end of this month.
