Show that four points with position vectors 4i^+8j^+12k^ , 2i^+4j^+6k^ , 3i^+5j^+4k^ and 5i^+8j^+5k^ are coplanar ?

For four vectors a,b,c,d to be coplaner , they should satisfy the condition of xa +yb+cz +td = 0
Here  x,y,z,t are scalars, and they should satisfy x +y+z+ t =0
So let t = 1
And on solving using the above conditions:

Hope you have got it.