[tex]\bf \begin{array}{llll}
term&value\\
\textendash\textendash\textendash\textendash\textendash\textendash&\textendash\textendash\textendash\textendash\textendash\textendash\\
t_1\\
t_2\\
t_3&18\\
t_4&18\cdot r\to 18r\\
t_5&(18r)r\to 18r^2\\
t_6&(18r^2)r\to 18r^3\\
t_7&(18r^3)r\to 18r^4=1458\\
\end{array}\qquad thus
\\\\\\
18r^4=1458\implies r^4=\cfrac{1458}{18}\implies r=\sqrt[4]{\cfrac{1458}{18}}
\\\\\\
now\implies t_n=t_1\cdot r^{n-1}
[/tex]
[tex]\bf \begin{cases}
t_1=\textit{first term}\\
r=\textit{common ratio}\\
t_n=n^{th}\ term
\end{cases}
\\\\\\
\textit{we know the 7th term is 1458, thus}
\\\\\\
t_7=t_1\cdot r^{7-1}\implies \cfrac{1458}{r^{7-1}}=t_1[/tex]
and just plug in there the "common ratio" or "r" you found, to get the first term
