I believe a = -1 works just by playing with it, but I don't know how to tell if that's the only solution or if there are others. . . anyone ?!
I'm sorry no one is replying to your threads :s
Hmm I'll try for this one. Seems easy
Two vectors are linearly dependent if there are m and n different from 0 such that mu+nv=0. ---> mu=-nv.
So you have to set a such that the coordinates in the matrices have a common ratio.
so now find a
Note : this is similar to finding a such that the matrix 2x2 you've got is non invertible (determinant = 0)