Question

The y-component of a projectile’s velocity is 12.1 m/s. When the projectile once again passes by the height from which it was launched, what is the y-component of its velocity? impossible to calculate without knowing the x-component 0 m/s 12.1 m/s -12.1 m/s

Answer 1

It's 12.1 m/s, assuming that's the launch velocity that's given.
For projectile motion, velocity's y-component is parabolic/quadratic. It's x-component is constant, so you don't need to know it.