Sliders for unknown constants

When an equation contains an unknown constant, Desmos offers a slider for it. Drag the slider and watch the graph change.

Example: $y = x^2 + kx + 9$ with $k = 6$. At $k = 6$ the parabola just touches the $x$ axis, which means exactly one solution. Drag $k$ and watch it cross the axis twice, or not at all.

• To add a slider yourself, type an equation with a new letter (like $k$) and click add slider.
• Click the slider's ends to set its range and step size.

On the test: "for what value of $k$ does the equation have exactly one solution / no solution?" Slide until the graph does what the question describes, then check with the discriminant if you have time.