You need to pass in the arguments to get_seconds() as hours, minutes, and seconds, then your code will work:
def get_seconds(hours, minutes, seconds):
return 3600 * hours + 60 * minutes + seconds
amount_a = get_seconds(2, 30, 0) # 2:30:00
amount_b = get_seconds(0, 45, 15) # 0:45:15
result = amount_a + amount_b
print(result)