Answer:
the simplified expression is -y.
Explanation:
Combining like terms, 8y-9y = -y.
Like terms are terms that have the same variable and exponent. In this case, both 8y and -9y have the same variable (y) and exponent (1). Therefore, we can combine them by adding their coefficients:
8y - 9y = (8 - 9)y = -y
Therefore, the simplified expression is -y.