Answer
$F=6.67\displaystyle \times 10^{-11}\cdot\frac{mM}{d^{2}}$
Work Step by Step
$y$ varies ${\bf directly}$ with $x,$ or $y$ is directly proportional to $x$,
if there is a nonzero number $k$ such that$ \ \ y=kx.$
The number $k$ is called the constant of proportionality.
$y$ varies ${\bf inversely}$ with $x,$ or $y$ is inversely proportional to $x,$
if there is a nonzero constant $k$ such that $\displaystyle \ \ y=\frac{k}{x}$
---
We have a combined variation:
$F=k(mM)$
and
$F=\displaystyle \frac{k}{d^{2}}$
The combined varion is written as
$F=k\displaystyle \frac{mM}{d^{2}}$
Given that $k=G=6.67\times 10^{-11},$ it means that
$F=G\displaystyle \frac{mM}{d^{2}}$
or
$F=6.67\displaystyle \times 10^{-11}\cdot\frac{mM}{d^{2}}$
