To solve the problem, we need to identify the pattern or logic behind the operations with the symbol \(\bullet\) given in the question. Let's analyze the provided relationships:

1. Given: \(5 \bullet = 20\). We can interpret this as an operation performed on 5 to get 20.
2. Given: \(7 \bullet = 28\). Similarly, an operation is performed on 7 to obtain 28.

We assume that there is some consistent arithmetic operation being applied. Let's look for possible patterns:

1. Observe that \(5 \times 4 = 20\). Hence, the operation could be multiplying 5 by 4.
2. Observe that \(7 \times 4 = 28\). This suggests the operation is to multiply the number by 4 consistently.

Based on the identified pattern, we hypothesize that \(x \bullet = x \times 4\).

Now, apply this rule to find \(9 \bullet\):

1. Using the rule, \(9 \times 4 = 36\).

Therefore, \(9 \bullet = 36\). Thus, the solution is consistent with the pattern observed, and the correct answer is 36.