Answer:
First, we know that the area of a rectangle of width W and length L is:
A = W*L
In the case of Roberto's plan, we can see that the length of the whole rectangle is:
L = 1.5ft + x + 1.5ft = 3 ft + x
And the width is:
W = 3ft + x + 3ft = 6ft + x
Then the area of the whole thing is:
A = (3ft + x)*(6ft + x)
This is what we wanted, a product of two polynomials that represents the area of Roberto's plot.
Now if we subtract the white square (is a square of sidelength x, then its area is A = x*x) we will get the area of the border;
The total area of Roberto's borders is:
Area of the border = (3ft + x)*(6ft + x) - x*x
= 3ft*6ft + 3ft*x + x*6ft + x^2 - x^2
= x*9ft + 18ft^2
