Final answer:
To simplify the expression, we combine the like terms by adding their coefficients.
Step-by-step explanation:
To combine the like terms and simplify the expression, we add the coefficients of the like terms. The like terms in the given expression are 9x² and -3x, and 8y² and 4y².
Adding the coefficients of 9x² and -3x gives us 9x² - 3x, and adding the coefficients of 8y² and 4y² gives us 12y².
So, the simplified expression is 9x² + 12y² - 3x.