/sci/ - Science & Math

So I'm supposed to express y as a linear combination of a1, a2, and a3, or show that it is not. I still don't understand what exactly does it mean for something to be a 'linear combination' of something else.
My current idea is that I create a matrix of a1, a2, a3, and then add in y as the 4th column and then see if it equals zero, like Ax = 0 except x is the y vector. Is this how I'm supposed to do it?