Answer
$l=9$ and $b=6$
Work Step by Step
Formula to calculate the area of a rectangle, $A= l \times b$
Here, $l= b+3$
Thus, $A= (b+3) \times b =b^2+3b$
or, $b^2+3b=54$
or, $b^2+3b-54=0$
This implies $b^2+9b-6b-54=0$
or, $(b+9)(b-6)=0$
Thus, $b=-9,6$; $A \neq -negative$
Hence, $l=b+3=6+3=9$ and $b=6$
