Code:
wheelSpeeds[MotorType.kFrontLeft_val] = xIn + yIn + rotation;
wheelSpeeds[MotorType.kFrontRight_val] = -xIn + yIn - rotation;
wheelSpeeds[MotorType.kRearLeft_val] = -xIn + yIn + rotation;
wheelSpeeds[MotorType.kRearRight_val] = xIn + yIn - rotation;
Positive should be clockwise. Left side will spin forward and right side will spin in reverse.