ST is a midsegment, which tells you that BD=2(ST) or ST=½BD. Use that to substitute the values and find ST.
BD=2(ST)
29+x=2(2x+31)
29+x=4x+62
29=3x+62
-33=3x
x=-11
Now that we know x, we can substitute that into 2x+31 to find ST
ST=2x+31
ST=2(-11)+31
ST=-22+31
ST=9
