Can you explain this one to me?
it looks very easy but i'm just not catching

That appears to be the square root and the cube root...so what you really have is:

(2^(1/2))*(2^(1/3))

Now the rule for multiplication of similar bases with different exponents is:

a^b*a^c=a^(b+c), so in this case:

2^(1/2)*2^(1/3) is equal to

2^(1/2+1/3)  which is equal to

2^(5/6)

The answer is part a: use