28 days
If you are looking for how many days are in four weeks then 28.
A. 620 is a multiple of 4
You can only add exponents if you are multiplying two of the same variable together. For example,

You take them away if you are dividing.
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In a polynomial equation, you can add together two of the same variable if they have the same exponent
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Or you can take them away in a similar fashion,
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But you cannot add two (or more) different exponents,
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(it does not get any simpler)
I hope this has answered your question, if not, leave a comment and I'll update the answer.
Answer: (13) 128 (14) 3 (15) 38 (16) 21
Step-by-step explanation:
13) (x+3)+49=180
14) 66+8x=90
15) (x+3)+49=90
16) 3x+117=180