Answer
Length $= 6x+15$
Work Step by Step
The given area is
$=18x^2+69x+60$
Factor.
$=(3x+4)(6x+15)$
Width $=3x+4$.
Formula for the area is
$=$ length $\times $ width.
Or we can write.
$\Rightarrow (3x+4)(6x+15)=$length $\times (3x+4)$
Divide both sides by $(3x+4)$.
$\Rightarrow 6x+15=$length