In $6x<(x+9)$ the expressions $6x{\textstyle \phantom{\rule{1em}{0ex}}}\text{and}{\textstyle \phantom{\rule{1em}{0ex}}}(x+9)$ are joined with < inequality sign. It is obtained when an expression is related with another expression using the inequalities sign ($<,>,\le ,\ge$).

That means, that $6x<(x+9)$ is an inequality.