Answer & Explanation #1
To answer your first query. You can write a function that takes two arguments and returns their sum like this:
#Python program to add two numbers using a function
def add_num(a,b): #function for addition
sum=a+b
return sum #return value
num1=25 #variable declaration
num2=55
print("The sum is",add_num(num1,num2)) #call the function
Answer & Explanation #2
As for your error, I believe it is an indent error in your first return statement. All you must do is indent the return statement.