Final answer:
Your friend currently has 29 bottles of red wine in his wine supply for the dinner party.
Step-by-step explanation:
To find out how many bottles of red wine your friend currently has in his wine supply, you need to add up the number of bottles of each type of red wine. Your friend has 13 bottles of Zinfandel, 9 bottles of Merlot, and 7 bottles of Cabernet. To get the total number of bottles of red wine, add these quantities together:
Total number of bottles of red wine = 13 + 9 + 7 = 29 bottles
Therefore, your friend currently has 29 bottles of red wine in his wine supply for the dinner party.