Geometry

We can write an equation to solve this problem. 

First, the equation for the number of diagonals in a polygon is $n(n-3)/2$ where n is the number of sides or vertices. 

Thus, we can write the equation

$n(n-3)/2=2n$

Now, we simply solve for n.

$n^2-3n=4n\\ n^2-7n=0$

Using the quadratic equation, we get

$n=\frac{7\pm \sqrt{(-7)^{2}-4\cdot 1\cdot 0}}{2\cdot 1}\\ $

From this, we get

$n=7\\ n=0$

So we have 7 sides. 

So 7 is our answer. 

Thanks! :)