From $(18,24)=6 \nmid 9$ we know there is no solution.
To solve the indeterminate equation (4), it is necessary (i) to find the greatest common divisor $g=\left(a_{1}, a_{2}\right)$, and determine whether $g \mid c$; (ii) if $g \mid c$, i.e., there is a solution, then try to find a particular solution $x_{1,0}, x_{2,0}$. According to Theorem 1 and Theorem 5 in § 3, we can use the method of successive division to find a particular solution. Below, we will introduce a direct algorithm through a specific example to determine whether the equation has a solution, and if it does, to find its solution. This algorithm is also applicable when $k>2$.