R
R
is the midpoint of line seqment PT¯¯¯¯¯¯¯
P
T
, and Q
Q
is the midpoint of line segment PR¯¯¯¯¯¯¯¯
P
R
. If S
S
is a point between R
R
and T
T
such that the length of segment QS¯¯¯¯¯¯¯
Q
S
is 13
13
and the length of segment PS¯¯¯¯¯¯¯
P
S
is 21
21
, what is the length of segment ST¯¯¯¯¯¯¯
S
T
?