v1 = (2 -1 2 1) v2 = (0 -1 1 -3) These vectors are column vectors.
Find a 2x4 matrix A, with linearly independent rows, such that Av1 = 0 and Av2 = 0.
Or, find a, b, c, d, e, f, g, h such that
and
That gives you four equations to solve for 8 values. Of course, there will be an infinite number of such matrices just as there are an infinite number of possible bases for the orthogonal space FernandoRevilla suggests.