Let's define the next variables:
x: number of Matt's DVDs
y: number of Dan's DVDs
Matt had 3 times more DVDs than Dan, then:
x = 3y
In total they had 68 DVDs, then:
x + y = 68
Substituting the first equation into the second equation:
3y + y = 68
4y = 68
y = 68/4
y = 17
Dan had 17 DVDs