Start by setting the denominator to zero to find the number you will divide by then set it up
The equation which represent total amount of income he earns from selling book is, 
Since, Selling price of one novel is n dollars
So, selling price of b books is, 
Since, he receives 9% commission on each book sale.
So, total commission he receives on selling of b books is,

And also he received a bonus of $50,000 to sign the contract to write the book,
So, total amount of income he earns is represented by A,

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