it will be helpful to remember the following:
we can say 2 triangles are similar if two angles of one triangle are equal to two angles of another triangle.
Having said that,
triangle pqr and qsr(or qsp) are similar
ang pqr = 90 = ang qsr
ang prq = ang qrs (they represent the same angle in the figure)
now because of that since corresponding sides are proportional,
qs/4 = 3/5
so qs = 12/5
Twixt, can you plz explain the concept mate?