#### Answer

The area of the land that can't be built on equals 6528 ft$^{2}$

#### Work Step by Step

To find the area of the land that cannot be built on we simply subtract the area of the smaller rectangle from the area of the larger rectangle.
Area of a rectangle: $A$ = $l$$w$
$A$$_{can't}$ = $A$$_{large}$ - $A$$_{small}$
$A$$_{large}$ = (172 ft)(124 ft)
$A$$_{large}$ = 21328 ft$^{2}$
$A$$_{small}$ = (148 ft)(100 ft)
$A$$_{small}$ = 14800 ft$^{2}$
$A$$_{can't}$ = 21328 ft$^{2}$ - 14800 ft$^{2}$
$A$$_{can't}$ = 6528 ft$^{2}$