$17-20$ Find a vector equation and parametric equations for the line segment that joins $P$ to $Q.$ need help on $18,19,20$

$17.P(2,0,0),Q(6,2,-2)$

$18.P(-1,2,-2),Q(-3,5,1)$

$19.P(0,-1,1),Q(\frac{1}{2},\frac{1}{3},\frac{1}{4})$

$20.P(a,b,c),Q(a,b,w)$