Final answer:
Antony's total debt can be expressed using the equation $6.50 - $3.75.
Step-by-step explanation:
To express Antony's total debt using negative numbers, we can use the subtraction operation to subtract the amount he borrowed from his friend ($6.50) from the amount he borrowed from his sister ($3.75).
The equation would be: $6.50 - $3.75
This equation represents Antony's total debt for that day.