A charge of +4 µC is placed at the origin O. A charge of −3 µC is fixed at A(4 cm, 0), and another charge of +5 µC at B(0, 3 cm). Calculate the magnitude and direction of the net electrostatic force on the charge at O. Give the direction as the angle below the +X-axis. (Take k = 9 × 10⁹ N m² C⁻².)
a) 211N,18∘ below +X-axis
b) 67.5N,72∘ below +X-axis
c) 211N,72∘ below +X-axis
d) 211N,72∘ above +X-axis