Rectangle Area: Express in Terms of Variables y and z with Dimensions 3y and y+3z

Let us begin by reminding ourselves of the formula to calculate the area of a rectangle: width X height

$S=w⋅h$

Where:

S = area

w = width

h = height

We must first extract the data from the sides of the rectangle shown in the figure.

$w=3y$$h=y+3z$

We then insert the known data into the formula in order to calculate the area of the rectangle:

$S=w⋅h=(y+3z)(3y)$

We use the distributive property formula:

$a\left(b+c\right)=ab+ac$

We substitute all known data and solve as follows:

$S=(y+3z)(3y)=(3y)(y+3z)$

$(3y)(y+3z)=(3y)(y)+(3y)(3z)$

$(3y)(y)+(3y)(3z)=3y^2+9yz$

Keep in mind that because there is a multiplication operation, the order of the terms in the expression can be changed, hence:

$(y+3z)(3y)=(3y)(y+3z)$

Therefore, the correct answer is option D: $3y^2+9yz$