#### Answer

The area of the rectangle is $9y^2-49z^2$ square units.

#### Work Step by Step

The area of a rectangle is given by the formula A=(length)(width).
The rectangle has a length of $3y+7z$ and a width of $3y-7z$, use the formula above to find the area and have:
$\\A = (3y+7z)(3y-7z)$
RECALL: $(a-b)(a+b)=a^2-b^2$
Use the formula above with $a=3y$ and $b=7z$ to have:
$\\A=(3y)^2-(7z)^2
\\A=9y^2-49z^2$