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:
