I found the solution in case anyone else has the same problem.

Lines intersect at origin O(0,0,0)

r: [t,2t,3t] = t[1,2,3] = t u

s: [3t,t,8t]=t[3,1,8] = t v

where

u = [1,2,3]

v = [3,1,8]

are the direction vectors of the lines

a vector which is normal to the plane containing r and s is

the cross product uxv

u x v = det([i, j, k; 1, 2, 3; 3, 1, 8])

u x v = 13i + j - 5k = [13,1,-5]

so the plane containing r and s has equation

13x + y - 5z = 0