In the given line, point S is on RT and

$$ Length\ of\ \overline{RS}=10cm $$

$$ Mid\ point\ of\ \overline{RS}=5cm $$

$$ Length\ of\ \overline{ST}=18cm $$

Then

$$ Distance\ between\ T\ and\ midpoint\ of\ \overline{RS}=18+5=23cm $$