Multiply eqaution 1 by -4 and add it to equation 3 to get
Now multiply the first of these equations by -1 and add it to the other to get
Now we factor the left hand side to get
Now lets check some values of a
Case I: a=4
we get 0=0 so the system is consistant dependant (has an infinite number of solutions.)
Case II: a=-4
We get 0=-8 So the system in inconsistant(has no solutions)
The system has exactly one solution and it is given above.
So the system is consistant independant.
I hope this clear it up.
Note this could have been done with a matrix and putting the matrix into eschelon form.