Answer
Multiply the second column by $-5$ and add it to the third column.
Work Step by Step
Given $$
\left|\begin{array}{rrr}{3} & {2} & {4} \\ {-2} & {1} & {5} \\ {5} & {-7} & {-20}\end{array}\right|=\left|\begin{array}{rrr}{3} & {2} & {-6} \\ {-2} & {1} & {0} \\ {5} & {-7} & {15}\end{array}\right|.
$$
In the L.H.S, multiply the second column by $-5$ and add it to the third column we get the R.H.S.
