The problem that I've been trying to figure out for the last couple hours:

When the parabola y = x^2 - a(x+1) +3 intersects the x-axis at one point,

then a= 1. ____ or 2. _____

Of course you set the y = 0

then while solving it comes to a point where I have to use the quadratic formula and comes out to:

a +- sqrt{(a+6)(a-2)}2

After that, I don't know what to do. How do I apply the statement 'intersects the x-axis at one point' to the formula?