On the attached diagram red curve is the graph of the quadratic function
As you can see this graph has no intersections with x-axis, this means that there are no solutions.
If you translate red curve 2 units down, you obtaine the graph (blue curve) of the function
and this graph has one common point with x-axis. This means that there will be exactly one solution.
Answer: k=-2 (you add -2 to the function
to obtain the function
).