Given that the total money Elise had, was $30.
Since she spent $18 on gifts and the rest on 3 corn dogs.
Let the cost of a corn god be 'x' dollars. Then the cost of 3 corn dogs will be,
[tex]\begin{gathered} \text{Expense on corn dogs}=\text{ No. of corn dogs}\times\text{ Cost of 1 corn dog} \\ \text{Expense on corn dogs}=3\times x \\ \text{Expense on corn dogs}=3x \end{gathered}[/tex]So it is found that Elise spent $(3x) on corn dogs.
Since she had spent all the money in buying these two things, the total exprense must be equal to the total money Elise had in the beginning,
[tex]\begin{gathered} \text{Expense on corn dogs}+\text{ Expense on gifts}=30 \\ 3x+18=30 \end{gathered}[/tex]Thus, the required equation is obtained.
Therefore, option (a) is the correct choice.