AHSME 1967 Problem 15

The difference in the areas of two similar triangles is $18$ square feet, and the ratio of the larger area to the smaller is the square of an integer. The area of the smaller triange, in square feet, is an integer, and one of its sides is $3$ feet. The corresponding side of the larger triangle, in feet, is: $\textbf{(A)}\ 12 \textbf{(B)}\ 9 \textbf{(C)}\ 6\sqrt{2} \textbf{(D)}\ 6 \textbf{(E)}\ 3\sqrt{2}$