Answer
The force on the 1.0 nC charge is $3.1\times 10^{-4}~N$ directed upward.
Work Step by Step
By symmetry, the horizontal component of each force on the 1.0 nC charge cancels out. The net force on the 1.0 nC charge is the sum of the vertical components of each force. Since positive charges repel each other, the net force is directed upward.
$F = 2\times \frac{(k)(1.0~nC)(2.0~nC)}{r^2}~sin(60^{\circ})$
$F = 2\times \frac{(9.0\times 10^9~N~m^2/C^2)(1.0\times 10^{-9}~C)(2.0\times 10^{-9}~C)}{(0.010~m)^2}~sin(60^{\circ})$
$F = 3.1\times 10^{-4}~N$
The force on the 1.0 nC charge is $3.1\times 10^{-4}~N$ directed upward.
