The area of a rectangle is the product of its dimensions:
[tex] A = w \cdot l [/tex]
where w and l are width and length, respectively. So, if we plug your values, we have
[tex] 8148 = 84l [/tex]
So, if you divide both sides by 84, you have
[tex] l = \dfrac{8148}{84} = 97 [/tex]
