How to know which equation of motion to use where?explain

We use v=u + at, when distance is not given

We use s=½(u+v)t, when acceleration is not given

We use v²=u² + 2as, when time is not given

We use s = ut +½at², when final velocity is not given

two charges are fixed in place on the x-axis as shown. the charge at x = 0 is negative and has magnitude q = 2 μcoulombs (1 μcoulomb = 10-6 coulombs). the charge at x = d, (d = +12 cm), is positive and has magnitude q = 4 μcoulombs. find a point x = xo along the x-axis at which the electric field is zero.

grav potential energy