Answer:
14
Explanation:
Since S is the midpoint of RT, then RS = ST
RS = RT - ST = (3x + 7) - (x + 7) = 3x + 7 - x - 7 = 2x
so we get that RS = 2x
but since RS = ST, and RS=2x & ST=x+7
then 2x = x + 7
2x-x = 7
x = 7
ST = x+7 = 7+7 = 14
P.S. Hope it makes sense. If you have any questions, feel free to ask them in the comments senction. I'll be happy to help. Have a wonderful day!