Solve for u.
Or: three vectors are co-planar if any one of them can be written as a linear combination of the other two.
Find a u such that a(i + j - k)+ b(2i - j + k)= ui - j + uk for some a and b. That is, a+ 2b= u, a- b= -1, -a+b= u. You can solve the first and second equations for a and u in terms of b, then find b so that the second equation is satisfied.