Answer:
Part A: The correct answer is Choice D: vertically upward.
Part B: [tex]5.68*10^{-6}N[/tex]
Explanation:
Part A:
Since the three charges all all negative, the force is repulsive, and therefore the force on the object at the top corner is directed upward.
Part B:
The force on the charge at top corner is due to the charges at the other two corners.
[tex]F_{32}+F_{31}=2\frac{kQ^2}{d^2} = 2*\frac{8.9*10^9*(2.5*10^{-9})}{0.14^2}=\boxed{5.68*10^{-6}N}[/tex]