Answer
He has 84 DVDs.
Work Step by Step
If $v$ is the number of movies on videotape, $2v$ is the number of movies on DVD.
$v+2v=126$ $\longrightarrow$combine like terms
$3v=126$multiplication property
$3v\div3=126\div3$ $\longrightarrow$simplify
$v=42$
$2v=2\times42=84$
