
Write it in the y=mx+b form.

The system will be inconsistent when the equations describe two parallel lines.
Two lines are parallel when their slopes are identical.

Also they can't be the same line, so the y-intercepts must be different.

So for every value of a and b such that ab=15 and a≠3/2 this system of equations is inconsisent.
It can be:

or

or

etc.