Do row operations to get echelon form.

Infinite solutions for values of a that give the form:

* * * | *

* * * | *

0 0 0 | 0

No solutions for values of a that give the form:

* * * | *

* * * | *

0 0 0 | Non-zero

Otherwise there's a unique solution.

This is a bit vague but you should understand what I mean if you understand this topic.