Show that the lines
`vec(r )=(3hat(i) -15hat(j) + 9hat(k)) + lambda(2hat(i) -7hat(j) +5hat(k))` and
`vec(r )=(-hat(i) + hat(j) + 9hat(k)) + mu (2hat(i) + hat(j) -3hat(k))`
do not intersect.