The length of ST you can calculate using Pythogoras.
If V is a point on ST such that RV is perpendicular to ST you could use coordinate geometry to find V.
If the coordinates of the vertices are, for instance, P(0,6), Q(9,6), R(9,0), S(0,0). You could find the slope and or the equation of the line through ST and the equation of the line perpendicular to ST and passing through R. The intersection of the two lines will give you V. There is a formula for finding the perpendicualr distance of a point from a line, you could use that too.
Hope that helps.