Algebra: A Combined Approach (4th Edition)

$(4x^{2}+8x+16)$ feet$^{2}$
We know that the area of a square is calculated as $side \times side$. We are given that the length of one side of the square is $(x+4)$ feet. Therefore, $Area=(x+4)(x+4)=x(x+4)+4(x+4)=(x)(x)+(x)(4)+(4)(x)+(4)(4)=x^{2}+4x+4x+16=(4x^{2}+8x+16)$ feet$^{2}$.