\[*\ \ :a^{7} \cdot a^{11} = a^{18}\]
\[*\ \ :a^{7} = a^{18}\ :a^{11}\]
\[*\ \ :a^{7} = a^{7}\]
\[*\ = a^{7} \cdot a^{7}\]
\[*\ = a^{14}.\]