Firstly it is easier to think of the power (3/4) as two separate parts. The first part being 3/1 (which is the same as cubing a number) and the second part being 1/4 (which is the same as the quartic root).
We should also think of the number in the bracket (16/81) as two separate numbers as well. For both 16 and 81, we should first find the quartic root and then after cube the number, and then place them back in the fraction to obtain our answer.
To demonstrate:16^(1/4) = 22^(3) = 8
81^(1/4) = 33^(3) = 27
Therefore, our answer is = 8/27 or 0.296 (to 3 d.p)