A
What have you tried?
I assume that you know the kinematic equations.
$\displaystyle x=x_{0}+v_{0}t+\frac{1}{2}at^{2}$
For each of the particles you are given the initial position $\displaystyle x_{0}$
Their initial velocity $\displaystyle v_{0}$ and their acceleration $\displaystyle a$.
Hint: for simplicity since Z is arbitrary and you are only care about distance from z you might as well put the origin of your coordinate system at z and so $\displaystyle z=0$.
Now write out each of the kinematic equations you will get a system of 2 equations in x and t.