I got these 3 questions from my homework, and I'm having a trouble time with them
** Two equally charged particles, held 3.2mm apart, are released from rest. The initial acceleration of the first particle is observed to be 7.22 m/s2 and that of the second to be 9.16 m/s2. The mass of the first particle is 6.31x10^7 kg. Find (a) the mass of the second particle and (b) the magnitude of the common charge.

Well, at first I tried to have this picture:
http://img718.imageshack.us/img718/4997/no4chp25.jpg
The thing that I got stuck on is that I guess since the question gives me the accelerations and that the 2 charges are at rest, there will be something to do with kinematics (or I should say, something like: v(f) = v(i) + at ???). But at the same time, I see that the question gives me one of the masses, the accelerations, and the distance already, can I use Coulomb's law to get the mass of the second charge?
I thought F12 = ma and F21 = m'a'. So then I just plug in the two in the equation of Coulomb's law to get m' ? Can I do that? Which direction is right?
** find (a) the horizontal components and (b) the vertical components of the resultant electric force on the charge in the lower left corner of the square (picture shown below). Assume that q = 1.13 µC and a = 15.2 cm. The charges are at rest.

Here is the picture of the question and my work:
http://img294.imageshack.us/img294/4297/no8chp25.jpg
but the book answers are (a) 2.34 N and (b)  0.642 N. So did my solving has errors somewhere?
** Two positive charges, each of 4.18µC, and a negative charge,  6.36 µC, are fixed at the vertices of an equilateral triangle of side 13 cm. Find the electrical force on the negative charge

Here is the picture of the question and my work
http://img827.imageshack.us/img827/1503/no9chp25.jpg
The same problem happens. The book answer is 24.5 N with the note after: "belong the angle bisector". I believe the book answer is right, but if that's the case, what wrong with my steps? I think the "electrical force on the negative charge", which is q1 in my picture, will be F31
Pls help me! >_< I really got stuck on these & know nowhere to get out Thanks