Answer:
12
Step-by-step explanation:
A² + B² = C²
9² + B² = 15²
81 + B² = 225
B² = 144
B = 12
Let's start with the fact that this is a function. Each element of Set A is paired with exactly one element of Set B.
This makes Set A the set of inputs and Set B the set of outputs.
There are a lot of other boxes to pick from, so I'm not sure what else you'd need to pick to make your answer correct.
The answer is obviously 100 that was so easy
