Find image of matrix (1 4, 2 5, 3 6) and find unitary matrix with similar image

Question

Let Do = (1 4, 2 5, 3 6] 3x2 matrix. Find D such that D* D = I and Im(D) = Im (Do)

furthermore, find D_ such that [D D_] is unitary matrix. D* = transpose conjugate

 

I have calculate and find that Im (Do) = Do but i'm stuck there.

thank you