what is 64 to the power of 2/3?

If a number is to the power of 2/3, you should think of it as to the power of 1/3 (so CUBE-rooted), and then being to the power of 2 (so squared).

So here, the cube root of 64 is 4, because 4x4x4=64. Then you do 4 squared, which is 4x4=16. So 64 to the power of 2/3 is 16!