Make Desmos try all four answers for you
Leave the unknown letter in the equation and drag a slider.
2:20 · Nonlinear Equations in 1 Variable and Systems of Equations in 2 Variables
Worked in this video
x² + kx + 9 = 0
Find k so the equation has exactly one solution. (A) 3 (B) 6 (C) 9 (D) 18
Type the equation and leave the k in. Desmos sees a letter it does not recognise and offers "add slider: k". Drag it. Exactly one solution means the parabola touches the x-axis once: at 3 it floats above, at 9 and 18 it cuts through, at 6 it just touches. Note that k = −6 also gives one solution — it simply was not among the choices.
Answer: k = 6 — answer B