For 6d and 7a, multiply the top and bottom by the denominator. 7b is a bit different since it is a cube root. A cube root is basically x^(1/3), so multiply the top and bottom by x^(2/3). Does that make sense?
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I have no clue how to do 6. d) or 7. a) and b)
I have not attempted anything because I don't know what to do, I was able to do the first 3 questions for 6 though!