Move robot towards reference axis

Hi! I'm an undergraduate currently working on a project where the goal is to control a robot with two DC-motors(no encoders) and an arduino board, towards a target by using visual data. I take an image with a webcam and then the robot needs to angle itself towards the target. My problem here is that the robot and the camera has two separate coordinate systems, so I need to calibrate the robot's angle towards the x-axis first, so that the readout from the angle-meter on the robot corresponds to the offset from the x-axis on the camera's coordinate system. I written a program that can find the xy-coordinates of the robot in the camera plane and I have a readout from the angle of the robot's angle-meter in MATLAB. How can I calibrate the robot so that it always has (approximately) a zero degrees readout from it's angle-meter when it is angled towards the camera's x-axis ?

My current code looks like this:

function[Offset] = CalibrateRobCam(ard, cam);
Frames = 1; %Number of frames to snap of plane.
Color = 1; % Red = 1, Green = 2, Blue = 3.
[X Y] = trackObject(cam, Color, Frames); %Gets coordinates of robot in the camera plane
X = round(X); Y = round(Y);
RobotPos = [X ;Y];

%%Calibrate Robot coordinate system with camera coordinate system
ZeroPos(ard); % Positions the robot for where it read euler angle approx zero degrees.
theta = readAngle2(ard); %reads robot angle from gyro/accelerometer. angle is [-180 180]
O = [cosd(theta) - X ; Y]; %Creates reference vector for positioning the robot towards x-axis. 
U = [cosd(theta); sind(theta)]; %Creates vector for robot, converting from polar to cartesian.
CosTheta = dot(O,U)/(norm(O)*norm(U)); %Angle in radians between vectors
Theta = round(acosd(CosTheta)) ; % Angle in degrees between robot and target
Drive3Angle(ard, -Theta, 5); %Steers the robot towards the target angle

Offset = readAngle(ard); %Robot angle-meter offset when it is positioned towards camera x-axis
end

This will not work as I intend it to do... the input variables "ard" and "cam" correspond to the arduino-serial connection and the webcam. Here's an illustration for clarification:

I need to find the angle "theta" and then steer the robot that amount of degrees. I would be very thankful if anyone could help me with this issue since I've been stuck for a long while on this problem now. I have access to almost every toolbox. Thanks!