Chapter 1 - Solving Linear Equations - 1.2 - Solving Multi-Step Equations - Exercises - Page 17: 36

$d=12\,ft$

$\text{Area}=\text{length}\cdot\text{width}$ $\implies 210=(d+9)10$ Using distributive property, we have $210=10d+90$ Subtracting $90$ from both sides, we get $120=10d$ $\implies d=\frac{120}{10}=12$ In units, $d=12\,ft$