Gravitational pull on two objects

[enantiomer1]

If three balls (all weighing 2.0kg) are lined in a row, what is the magnitude of the net gravitational force on mass A due to masses B and C in the figure. B is 10cm from A, and C is 50 cm from A (and of course C is 40 cm from B)
(here's a 'diagram')
(A)----(B)----------------------------C
I've been using the equation G*(m1m2/Rsq) and been adding the gravitational pull from a & b and a & c but for some reason that's wrong. So where do I need to fine tune my equation?

 

[D H]

What answer do you get? What do you expect to get (i.e., why do you think you are wrong)? Show your work, please.

 

[enantiomer1]

I'm expecting to get 3.00×10^-9 due to, the fact that G*(2kg*2kg)/.1m + G*(2kg*2kg)/.5m = 3.00×10_-9

 

[D H]

You aren't using Newton's formula for gravitation. Look the equation, then look at what you are doing.