In math the sum means to add
Answer:

Step-by-step explanation:
It's already in a form that only ha positive exponents. Just to state it though you could rewrite it as
or
The question asks for positive exponents though so yu don't need to do that.
You also need to know
so in this case 
so 
Answer:
I believe it should be b and d
Step-by-step explanation:
hoped it helped :)
Answer:
length = 13
Step-by-step explanation: