Answer:
![3\sqrt[3]{x-3}-5](https://tex.z-dn.net/?f=3%5Csqrt%5B3%5D%7Bx-3%7D-5)
Step-by-step explanation:
You can factor out the common term of 27 from 81*
![\sqrt[3]{27(x-3)}-5](https://tex.z-dn.net/?f=%5Csqrt%5B3%5D%7B27%28x-3%29%7D-5)
Separate
![\sqrt[3]{27}\sqrt[3]{x-3}-5](https://tex.z-dn.net/?f=%5Csqrt%5B3%5D%7B27%7D%5Csqrt%5B3%5D%7Bx-3%7D-5)
Simplify
**
![3\sqrt[3]{x-3}-5](https://tex.z-dn.net/?f=3%5Csqrt%5B3%5D%7Bx-3%7D-5)
Finito. Nothing further can be done
*factor out 27

**
because

You can basically plug in the numbers for x and y into the options:
A. y/x = 3
12/4 = 3, so this one is right
B. xy = 3
12*4 = 48, so this is wrong
C. x = 3y
3*12 = 36, which is not x (4), so this is wrong
D. y = 3x
3*4 = 12, which is y (12), so this is right
So, A and D are right
X letter is formed by two lines. The minimum amount of cubes that one side may have to look a line that cross other line is 3, but given that one cube is shared by the two lines you only need 5 cubes. So the answer is that the fewest unit cubes that Peter can use to build the figue in the shpe of X is 5 cubes.