Any time you have free variables in an expression, the calculator will offer to let you define them with sliders. You will see the option to add sliders appear in the expression bar once you finish typing in your equation.
You can also use the same variables in several expressions to plot curves that will change together.
In this example, the value of c defines two parallel lines that move up and down together.
In this example, the two lines stay perpendicular to each other for any value of m.
To adjust the limits and interval of your slider, click either of the values at the ends of the slider bar. Input your values and click "set to complete the adjustment.