Velocity of a car traveling in a straight line increases from 0 m/s to 30 m/s in eight seconds what is the average acceleration of the car

Average acceleration is how much the car increases (on average) per second.  So, since it increases by 30 m/s in 8 s, then dividing 30 by 8 will give you an average acceleration of 3.75 m/s^2 (remember that acceleration is ever increasing, so the unit is s^2, not just s)

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.