Question

find the diameter of a circle 2 circumference is 66

Answer 1

FULL ANSWER WITH STEPS

Circumference of the circle = 66
Let the radius of the circle be $r$. 
Then, diameter of the circle = 2 × Radius = $2r$

Now, 
$Circumference = 2 \pi r$
⇒ $2 \pi r = Circumference$
⇒ $2*\frac{22}{7}*r=66$        [I am taking $\pi$ as $\frac{22}{7}$. ]
⇒ $\frac{44}{7}r=66$
⇒ $\frac{44}{7}r*\frac{7}{44}=66*\frac{7}{44}$    [Multiplying both sides by $\frac{7}{44}$. ]
⇒ $r=\frac{21}{2}$

Now, 
Diameter = 2 × Radius
⇒ Diameter = $2r$ = $2*\frac{21}{2}$ = $21$

∴ The diameter of the circle is of 21 units. 

Hope this may help you. 

If you have any doubt, then you can ask me in the comments.