Imagine that x is the number of months that Martha has worked before earning 27,000 dollars. We also know that since the $9,000 is also included in the earning of $27,000, we will be adding $9,000 to however the rest of the equation. Since she earns $4,500 per month, and she works for x months, we can multiply these two together to form the money earned (without bonuses) after x months, 4,500x. Now we can make our equation.
$9,000 + 4,500x =$27,000
4,500x = $18,000 Subtract $9,000 to get x on one side
x = 4 months Divide by 4,500 to get x by itself