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