we are given
A rectangle is bounded by the x-axis and the semicircle y=sqrt(36-x^2)
we can see that
length=L=2x
width=W=y
we know that
area of rectangle is length*width
[tex]A=L*W[/tex]
now, we can plug values
[tex]A=2x*y[/tex]
[tex]A=2x\sqrt{36-x^2}[/tex]..............Answer