Help please, Four charges are placed at the four corners of a square of side 15 cm.The charges on the upper left and right corners are +3 uC and -6 uC respectively. The charges on the lower left and right corners are -2.4uC and-9 uC respectively. The net force on -6uC charge is?
25N, 78 degrees above the -ve x-axis
13N, 31 degrees below the +ve x-axis
18N, 75 degrees above the +ve x-axis