The force of gravity on Planet A is 5.7 msec2. How much would an object with a mass of 5 kg weigh on Planet A?

Okay, first up: The question is poorly Worded. Something with units of meters per second per second is not a "force", it's an acceleration. That might be a source of confusion. In the SI, force is measured in units of kilogram-meters per second per second (aka newtons), which should be a BIG clue as to how to solve the problem.

To spell it out:

Newton's second law is that force = mass * acceleration. You know the mass. You know the acceleration. You're looking for the weight (which is a force). The rest is basic arithmetic.