Intersection of two surfaces

*Note:* This is from a Vector Calculus course, so I hope it belongs in this section.

**The question:**

For what values of 'a' do the surfaces and not intersect?

**My attempt:**

I rearranged the first equation like so:

Then equated it with the other:

Usually I'd solve for z to find the conditions for the intersection, but we have two variables now.

I then solved for a:

So when z = -1, 'a' isn't defined. I then substituted this value into the very first equation to see what happens, but I just get the origin point.

I don't think I'm doing this correctly. Any assistance would be great!