$$
  P^2 = a^3
$$

$$
  P^2  =  {{4 \pi^2} \over { k^2 (M_{\rm Sun} + M_{\rm Earth}) }}  a^3
$$

$$
  P^2  =  {{4 \pi^2} \over { k^2 (M_{\rm tot}) }}  a^3
$$

$$
  k^2  =  {{4 \pi^2} \over { P^2 (M_{\rm tot}) }}  a^3
$$

\bye
