Since that is an eyeball problem, perhaps you have shown your work already.
"Undefined" slope? That's a bit odd.
Normally,
Horizontal lines have 0 (zero) slope.
Vertical Lines have "No" slope. This may be what you mean by "undefined".
If so, x = -6, and you are done.
If there is a "Standard" form, it may A*x + B*y = C, where A = 1, B = 0, and C = -6