Before we begin, let's cross out "the small wheel could have an 18 inch diameter". This is useless information and should be disregarded to avoid confusion. Lots of questions include extraneous information to make you second guess yourself, so it is important to be aware of what the question is asking you and what you need in order to answer it.
The formula for circumference is
x diameter. With this formula, we can substitute the value of the diameter, 52, into the equation.
Circumference = 3.14 x 52
Circumference = 163.28
Final Answer: the circumference of the big wheel is 163.28 inches