Okay, let's define variables.
Basketball starts with a "b" so let's call it the variable "b."
Softball starts with an "s" so let's call it the variable "s."
Now the question asks us to write an expression to represent the circumference of a basketball. So essentially, the answer has to be "b = (something), right?"
Now since we know that the circumference of the basketball is three times that of a softball, we can say that "b (circumference of the basketball) = 3 x s (circumference of the softball)."
Answer: b=3s