Question

A uniform electric field is directed upward and has a magnitude of 5 N/C. What are the magnitude and direction of the force on a charge of -4 C placed in this field?

Answer 1

Here you know the Electric field, so no need for Coulomb's, just multiply times the charge. Remember what the signs mean.
$$
F=qE
$$
It helps to remember that the units for E is Newtons per Coulomb, so when you multiply E by q you get Newtons (i.e. force). The units for E is also Volts per meter -- that comes in handy sometimes.