- When you compute a product of the form $(a\cdot 10^n)(b\cdot 10^m)$ all you have to do is to compute $ab$ first, and then $10^n\cdot 10^m$ using the property: $10^m\cdot 10^n=10^{m+n}$. If $ab$ is greater than $10$ or less than $1$ then it is recommended to write it as a number $p$ such that $1\leqslant |p|\lt10$ times a power of $10$ and then repeat the same procedure we did. In this particular example: $ \begin{align}(-2.3\times10^{13})\cdot(0.8\times 10^{-28})&=[(-2.3)\times(0.8)]\times[10^{13}\times 10^{-28}]\\ &=(-1.84)\times(10^{13-28})\\ &=-1.84\times 10^{-15}. \end{align}$
- To prove that $\rm\frac{2GMr}{D^3}=\frac{Gm}{r^2}\iff D=\left(\frac{2M}m\right)^{1/3}r$ you really have to be very fluent at algebra. For that particular case, you have to multiply both sides by $\rm D^3$ then regroup all the extra terms in the RHS and then take the cubic root to eliminate that exponential power. If you don't know how to do this then you surely have to revise your algebra before taking any astronomy courses.
As Gerry said: You're going to need to find a refresher or bridging course somewhere, and do that before you try the astronomy course. Or in the case of self-study, go look for some good intros to algebra, precalculus, and calc.
I hope this helps.
Best wishes, $\mathcal H$akim.