Independent, vectors V1 and V2 determine a plane. If the vector Y is to be in that plane, then it has to be a linear combination of V1 & V2. Thus for constants a & b, the following holds true:
aV1 + bV2 = Y, or a(1, 2, -1) + b(-2, -1, 1) = (4, -1, h) , so a separate equation for each component must hold:
a -2b = 4
2a -b = -1
-a +b = h , solve them for a, b and h: 3 equations & 3 unknowns.
If we add equations 2 & 3, then b drops out and a = h - 1
Substitute this expression for a into equation #1, simplify and get b = (h - 5)/2
But h = (b - a) from equation 3, so get h = (h - 5)/2 - (h - 1) = -h/2 -3/2 so (3/2)h = -(3/2) and h = +1
Hence Y = (4, -1, +1) is a vector that lies in the plane.