Solution

For a circular orbit, the gravitational force provides the centripetal force
$\frac{GmM}{r^2}=\frac{mv^2}{r}$
Cancel m and multiply by r
$\frac{GM}{r}=v^2$ (1)
The kinetic energy is
$K=\frac{mv^2}{2}$
$v^2=\frac{2K}{m}$
And subsituting into equation (1)
$\frac{GM}{r}=\frac{2K}{m}$
we get
$2K=\frac{GmM}{r}$
The potential energy is
$U=-\frac{GmM}{r}$ , so
$U=-2K$