Lets say w is the width of the flower bed. We can make an equation to find the area.
The length of the of the flower bed is (w+5) and the width is w.
To find the area we need to multiply the length(w+5) by the width(w) and set it equal to 84,
(w+5)(w)=84
w^2 +5w=84
Using the quadratic equation you get the solutions of 7 and -12. Since 7 is positive it is our width. Our length is 7+5 or 12.