I have inserted a generic quadratic into the TI-84.
In this instance I entered #x^2-2x-6# by using the keys in the following order.
First I pressed the x, t, theta, n key for x.
Next I pressed the #x^2# key.
Next I entered a subtraction followed by 2 then the x, t, theta, n key again.
Finally, I entered a subtraction key followed by a 6.
Lastly, I pressed the grey "graph" button on the top right.