\(3+6+9+...+x=693\)
\(\Rightarrow\frac{\left(x+3\right).\left[\left(x-3\right):2+1\right]}{2}=693\)
\(\Rightarrow\left(x+3\right)\left[\left(x-3\right):2+1\right]=1386\)
\(\Rightarrow\left(x+3\right)\left[\left(\frac{x}{2}-\frac{3}{2}\right)+1\right]=1386\)
\(\Rightarrow\left(x+3\right)\left[\left(\frac{x-3}{2}\right)+\frac{2}{2}\right]=1386\)
\(\Rightarrow\left(x+3\right)\left(\frac{x-1}{2}\right)=1386\)