so the legnth (L) is equal to (=) 21 feet more than (21+) the width of the garden (W) or
L=21+W
the area equation is L times W so
LW=A
then you could subsitute L=21+w for L in the area eqation and get
W(21+W)=A
W^2+21W=A
and that is the area in terms of W