Final answer:
To find the coordinates of B' after a dilation of triangle ABC with a scale factor of 3, the coordinates of B (x, y) are multiplied by the scale factor, resulting in (3x, 3y). The correct answer is D. (12, -15).
Step-by-step explanation:
To find the coordinates of B' after a dilation of triangle ABC with a scale factor of 3, we multiply the coordinates of B (x, y) by the scale factor.
So, the coordinates of B' are (3x, 3y).
Therefore, the correct answer is D. (12, -15).