Question 1013807
First find the perimeter of just
the semi-circular top of the window
{{{ P[1] = (1/2)*2*pi*(x/2) }}} ( half of the perimeter of the circle )
{{{ P[1] = pi*(x/2) }}} 
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Now subtract this result from {{{ 21 }}} ft
{{{ 21 - pi*(x/2) }}}
This result is twice the length of the rectangle
plus the width.
Let {{{ y }}} = the length of the rectangle
{{{ 21 - pi*(x/2) = 2y + x }}}
{{{ 2y = 21 - pi*(x/2) - x }}}
{{{ y = 21/2 - pi*( x/4 ) - x/2 }}}
{{{ y = 21/2 - pi*( x/4 ) - (2x)/4 }}}
{{{ y = 21/2 - ( pi + 2 )*x/4 }}}
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The area of the rectangular part is:
{{{ A[1] = x*y }}}
{{{ A[1] = ( 21x )/2 - (( pi + 2 )/4 )*x^2 }}} 
The area of the semi-circle is:
{{{ A[2] = (1/2)*pi*( x/2 )^2 }}}
{{{ A[1] + A[2] =  ( 21x )/2 - (( pi + 2 )/4 )*x^2  + (1/2)*pi*( x/2 )^2 }}}
{{{ A[1] + A[2] =  ( 21x )/2 - (( pi + 2 )/4 )*x^2  +( pi*x^2 ) / 8 }}}
{{{ A[1] + A[2] =  ( 21x )/2 - (( 2*pi + 4 )/8 )*x^2  +( pi*x^2 ) / 8 }}}
{{{ A[1] + A[2] = ( 21x )/2 - (1/8)*( pi + 4 )*x^2 }}}
Hope I got it!