Chapter 3.2, Problem 20E

Find the determinants in Exercises 15–20, where

$\left|\begin{array}{ccc}\text{a}& \text{b}& \text{c}\\ \text{d}& \text{e}& \text{f}\\ \text{g}& \text{h}& \text{i}\end{array}\right|=7.$

20. $\left|\begin{array}{ccc}\text{a}& \text{b}& \text{c}\\ \text{d+}\text{3}\text{g}& \text{e+}\text{3}\text{h}& \text{f+}\text{3}\text{i}\\ \text{g}& \text{h}& \text{i}\end{array}\right|$