To find the area of a rectangle, you can use the formula:
\[ \text{Area} = \text{length} \times \text{width} \]
Given the dimensions of the rectangle as 5 ft and 18 ft, we can substitute these values into the formula:
\[ \text{Area} = 5 , \text{ft} \times 18 , \text{ft} \]
Calculating the area:
\[ \text{Area} = 90 , \text{ft}^2 \]
So, the area is:
\[ a = 90 , \text{ft}^2 \]