To solve for time in a projectile motion problem, you typically use one of the kinematic equations of motion. For example, if you are given an equation like $y = v_0 t + \frac{1}{2} a t^2$ (where $y$ is the vertical displacement, $v_0$ is the initial velocity, $a$ is the acceleration due to gravity, and $t$ is time), you would solve for time $t$. Since the equation is quadratic in form (due to the $t^2$ term), you often use the quadratic formula or completing the square to find $t$.

So, the correct answer for solving for time in a projectile motion problem would be:

Quadratic equation or completing the square.

Would you like more details on how the quadratic formula works in this context?

Here are 5 related questions:

1. What is the general form of a quadratic equation?
2. How does gravity affect the motion of a projectile?
3. What are the key differences between solving quadratic and linear equations?
4. How do you use the quadratic formula to solve for time in projectile motion?
5. Can completing the square always be used in a quadratic equation?

Tip: In projectile motion problems, the vertical and horizontal components are often treated separately, so it's important to break them down before solving for variables like time.