The area of the square is given by:
[tex]A=s^2[/tex]Where s is the length of the side. Then s=(2x+1) km.
By replacing this into the formula we have:
[tex]A=(2x+1)^2[/tex]Also, the square of a sum is given by:
[tex](a+b)^2=a^2+2ab+b^2[/tex]If a=2x and b=1, then:
[tex]\begin{gathered} (2x+1)^2=(2x)^2+2(2x)(1)+(1)^2 \\ (2x+1)^2=4x^2+4x+1 \end{gathered}[/tex]Thus, the area of the square in terms of the variable x is:
[tex]A=4x^2+4x+1[/tex]