Question

which chemical formula is correct? Ag9CH3CO2)2, K2(CN), Fe(OH)2, or MgHCO3

Answer 1

Your Ag9CH3CO2)2 formula has a type, so it is clearly wrong. If you drop the other formulas into Wikipedia, you will find that iron(II) hydroxide is the only one with a page.
http://en.wikipedia.org/wiki/Iron(II)_hydroxide

Alternatively, if you are working out of a textbook, you could find the table of common ions and match the charges of the cation and the anion. If the charges do not balance (and no net charge is indicated), then the formula is wrong. Here is an example of a good formula:\[\text{Calcium cation }\rightarrow Ca^{2+} \space (N_3)^-_2\leftarrow\text{Azide anion} \quad\bigg|\quad (+2)+2(-1)=0\]
And an example of a broken formula (there should only be one azide):\[\text{Potassium cation }\rightarrow K^{+} \space (N_3)^-_2\leftarrow\text{Azide anion} \quad\bigg|\quad (+1)+2(-1)=-1\]

Answer 2

\[Fe(OH)_{2}\]
since,
\[Fe ^{2+} + (OH)^{-} \rightarrow Fe(OH)_{2}\]