You know that the lines PT and ST are perpendicular.
By definition, the slopes perpendicular lines are opposite reciprocal. This means that if the slope of a line is:
The slope of a perpendicular line to that line is:
Knowing that:
You can find the slope of the line PT using this formula:
Where two points on the line are:
In this case, you can set up that for the line PT:
Then, substituting values into the formula and evaluating, you get:
Knowing the slope of PT, you can determine that the slope of ST is:
Hence, the answer is: