Check the picture below.
[tex]\cfrac{18}{18+x}~~ = ~~\cfrac{19.2}{38.4}\implies \cfrac{18}{18+x}~~ = ~~\cfrac{19.2}{2(19.2)}\implies \cfrac{18}{18+x}~~ = ~~\cfrac{1}{2} \\\\\\ 36=18+x\implies 18=x[/tex]
