We have been given that Maria put trim around a banner that is the shape of a triangle. Each side is 18 inches long.
First of all we will find the perimeter of our given triangle.
We know that perimeter of a triangle is sum of its three side lengths. Since each side of triangle has same measure, so its perimeter would be 3 times its side length.

We know that 1 feet is equal to 12 inches.
To convert 54 inches into feet, we will divide 54 by 12.

Since Maria has
foot of trim left, so we will add it to perimeter of triangle to get the length of trim when she started.

Therefore, the length of trim was 6 feet, when Maria started.