## Elementary Linear Algebra 7th Edition

$$\left|\begin{array}{rrr}{3} & {8} & {-7} \\ {0} & {-5} & {4} \\ {0} & {-15} & {20}\end{array}\right|=-120.$$
Given $$\left|\begin{array}{rrr}{3} & {8} & {-7} \\ {0} & {-5} & {4} \\ {6} & {1} & {6}\end{array}\right|.$$ Adding $-2$ times the first row to the second row, we get $$\left|\begin{array}{rrr}{3} & {8} & {-7} \\ {0} & {-5} & {4} \\ {0} & {-15} & {20}\end{array}\right|.$$ Now, we have $$\left|\begin{array}{rrr}{3} & {8} & {-7} \\ {0} & {-5} & {4} \\ {0} & {-15} & {20}\end{array}\right|=3\left|\begin{array}{rrr} {-5} & {4} \\ {-15} & {20}\end{array}\right|=-120.$$