Answer:
[tex]a=7,\\b=3[/tex]
Step-by-step explanation:
The area of a rectangle is equal to [tex]A=l\cdot w[/tex], where [tex]l[/tex] is the length of the rectangle and [tex]w[/tex] is the width of the rectangle. In a rectangle, opposite sides are equal.
Therefore, we can write the following system of equations:
[tex]\begin{cases}a\cdot (9+b)=84,\\a\cdot b=21\end{cases}[/tex]
Expand the first equation:
[tex]9a+ab=84[/tex]
Substitute the second equation into the first ([tex]ab=21[/tex]):
[tex]9a+21=84,\\9a=63,\\a=\boxed{7}[/tex]
Solving for [tex]b[/tex]:
[tex]7b=21,\\b=\frac{21}{7}=\boxed{3}[/tex]
