Answer:
It's choice G or H
Explanation:
The cube and the disc are representative of the variable r and a 1 (a single unit) The question says "Using the representations above..." Hopefully right before this question it tells you (above) which is the variable and which is the single unit.
The left and right sides of the balance are going to represent the left and right sides of the equation, with the = equal sign being the middle.
The 2 on the left side and the 18 on the right side will need to have the same image representing it on both sides (2 on the left, 18 on the right) because they are both numerical values. Then, there should also be one shape on the left that stands for the variable. Without seeing the info that preceded the question, it is either Answer G or Answer H. Edit in the additional info. I will be able to say exactly which one. But I bet you can see it!