How to solve 51 code project?(Python)

Question

I can't know that.
This is my code.
What is arror?
class Juice:
def __init__(self, name, capacity):
self.name = name
self.capacity = capacity

def __str__(self):
return (self.name + ' ('+str(self.capacity)+'L)')
def __add__(Orange, Apple):
return (Orange + Apple
a = Juice('Orange', 1.5)
b = Juice('Apple', 2.0)

result = a + b
print(result)