Triangle xyz is reflected across the line mc013-1.jpg, and x’ = (2, –15). what are the coordinates of x?

The image is not provided, therefore for your understanding, I will assume that the image has been reflected across y-axis (condition taken from a similar question).
There are certain reflection rules across whatever you want to reflect. In this case, we want to reflect the shape across the y-axis.
When you reflect a point across the y-axis, y coordinate remains the same, but the x-coordinate is transformed into its opposite, means the sign of the x coordinate will be changed from positive to negative or negative to positive.
Thus,
(x , y)→(-x , y)

X’ = (2, –15) is the point given to you, which is the reflected point. To find the pre-image, consider the rule described above.

X’ = (2, –15) → X = (-2 , -15)

In case, the question says it is reflected over x-axis then we would use another rule which says When you reflect a point across the x-axis, the x-coordinate remains the same, but  y-coordinate is transformed into its opposite, means the sign of the x coordinate will be changed from positive to negative or negative to positive.
Then,
X’ = (2, –15) → X = (2 , 15)