To obtain a balanced equation you need to be somewhat familiar with the mechanism of the reaction. It can be broken down into 3 steps:
1) Nucleophilic addition of hydride (by borohydride) to the carbonyl carbon of fluorenone
C13H8O + BH4- ==> C13H9O- + BH3
2) Complexation of BH3 with ethanol
BH3 + CH3CH2OH ==> CH3CH2OHBH3
Note that in a Lewis structure the product should appear with a formal positive charge on O and a formal negative charge on B.
3) Formation of product (proton transfer)
C13H9O- + CH3CH2OHBH3 ==> C13H9OH + CH3CH2OBH3-
Next, the ethoxyborohydride formed here can react with 3 additional equivalents of fluorenone, thereby picking up additional ethoxy groups in the process.
Adding these steps together and canceling like terms, we arrive at:
4C13H8O + BH4- + 4CH3CH2OH ==> 4C13H9OH + B(OCH2CH3)4-
