Final answer:
The area of the rectangle when x equals 3 feet is 84 square feet, corresponding to option d.
Step-by-step explanation:
To find the area of a rectangle described by the algebraic expression 6x² + 9x + 3 when x = 3 feet, we substitute 3 for x in the expression and perform the calculation:
Area = 6(3)² + 9(3) + 3
Area = 6(9) + 27 + 3
Area = 54 + 27 + 3
Area = 84 square feet
Therefore, the area of the rectangle when x = 3 feet is 84 square feet, which corresponds to option d.