Answer:
-15 + -3 = -18
Rob owes his father $18.
Explanation:
Rob has to pay back the same amount of money that he took. Since he took 15 dollars, he has to give back 15 dollars which makes the 15 negative. The same goes for the 3 in the equation.
To find out the total amount Rob has to pay back, we have to add -15 and -3, which is -18.