I think that there is a number missing here: \(\frac{2}{6}\)
in the sense that\(\frac{2}{6}\) of the 24 members alreaday support the idea.
\(\frac{2}{6}\) of 24 is
\(\frac{2*24}{6}=8\)
which means that 8 members already support the idea.
So if they need the support of 13 members, this means that they need 13-8=5, that is 5 more members.