Answer
$$
\left|\begin{array}{rrr}{3} & {8} & {-7} \\ {0} & {-5} & {4} \\ {0} & {-15} & {20}\end{array}\right|=-120.
$$
Work Step by Step
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 $$
\left|\begin{array}{rrr}{3} & {8} & {-7} \\ {0} & {-5} & {4} \\ {0} & {-15} & {20}\end{array}\right|=3\left|\begin{array}{rrr} {-5} & {4} \\ {-15} & {20}\end{array}\right|=-120.
$$
