reverse solving of formula

Hi folks. I am trying to draw a regular hexagon given a diameter (being the diameter of a circle drawn around the hexagon).

I basically have this formula where a = the length along the 6 sides of the hexagon:

d = a + 2 * cos60 * a (this is right, right?)

So what I am trying to figure out is how to solve for a. I got this far but my old brain is having a hard time with it:

d - a - 2 * cos60 * a = 0,

d - 2 * cos60 * a = a,

now I think I am supposed to divide the left side by a again but that wouldn't make sense since then I am also dividing the right side by a which would give me 1. Thanks for any pointers.

J