Let me give you a hint as to how to solve problems like this. Break down the units to their fundamentals. By doing this you can look a the units you have an why units you need to get to. This will then tell you the basic equation to get to your solution. Let me give you an example. I do a 100 meter sprint in 6 seconds. What is my average velocity for the sprint. To figure this out I would first need to remember the velocity is given in terms of some distance over time. So, the most convenient units to use for this would be meters over seconds or m/s. Looking at it like this, I know that I have to divide the distance traveled by the time it took to cover the distance in order to get the velocity. So, the equation would be: 100 meters / 6 seconds = 16.666667 meters/second. So, to get the answer to your problem, you need to figure out to things. What are the basic units that collectively are a Newton and what are the units of acceleration. The Newton is not a fundamental unit. It is a combination of other units that we call a Newton so that we don't have to write out all of the others. Look up these other units and then figure out what mathematical operations you need to do with them and mass to cancel out units until all that you have left are the units of acceleration. Then performs those mathematical operations on the numbers. Once you look at it this way, it really is not hard to figure out the answer.

F = m.a F = 400 N m= 70 kg a =? 400 = 70*a a = 400/70 a = 5.71 m/s2

I just wanted to say that alkaabi is correct thats your answer to your uestion. Sometimes its good to know just rememebr F=mass*acceleration. If you ever need more help I used to tutor college physics and I am getting my Ph.D in chemical and material physics, my e-mail is Nickykat1985@yahoo.com. Hope the hw goes well.