We have formual for area of rectangle A=l*w where l is length and w is width. We can transform it to obtain width
A=l*w /:l (divide both sides by l)
[tex] \frac{A}{l}=w [/tex]
Knowing that A=189 and l=21 we can substitute our data to the formula
w=[tex] \frac{189}{21}= 9[/tex] - its the result