Final answer:
The expression -√9y^2 can be simplified to 3y.
Step-by-step explanation:
To simplify -√9y^2 when y < 0, we first simplify √9y^2.
Since we're taking the square root of a squared term, the square root and the square will cancel out, leaving us with the absolute value of the original term.
We know that √9y^2 is equivalent to |3y|, because the square root of 9 is 3 and the square root of y^2 is the absolute value of y, which is |y|.
However, since we are given that y < 0, |y| becomes -y, and thus √9y^2 simplifies to 3(-y), or -3y.
We then apply the negative sign from the original expression, giving us -(-3y), which simplifies to 3y.