Translated to notation, this question is asking you to solve for n when:
[tex] \frac{8.4\times10^9}{n}=5.6\times10^{27} [/tex]
A good first step would be to multiply both sides by n, cancelling out the n in the denominator on the left side and leaving you with:
[tex]8.4\times10^9=(5.6\times10^{27})n[/tex]
And of course, to get n by itself from here, you simply divide both sides by [tex]5.6\times10^{27}[/tex]:
[tex]n= \frac{8.4\times10^9}{5.6\times10^{27}} [/tex]
