Answer:
Length of ST = mQR/2 = 4
Explanation:
S and T are the mid-points of PQ and PR, so ST is parallel to and equal to half the length of QR.
(following corrected due to mistake)
Length of QR = sqrt((3--5)^2+(4--2)^2)=sqrt(8^2+6^2)=10
Length of ST = mQR/2 = 5
I apologize for the error previously committed.