Consider the equation for the circumference of a circle, 2πr. Notice that this can also be written as 2rπ.
Recall that the diameter of a circle is equal to twice the radius, or
d = 2r
And make a substitution into the original equation, giving us the circumference c = πd. We know the circumference is 5.2mm; plugging in, we get
5.2 = πd.
Note: π is NOT a variable, it represents a number. Therefore, we can solve the equation for the diameter d = c / π, or
d = 5.2 / π.
I'll let you crunch the number :D
I hope this helps!