Our robot base has only 2 degrees of freedom: a translation along the x axis and rotation along the z axis. Our robot cannot move instantaneously in the y axis due to the fixed steering wheel assumption. Since our robot moves only on the ground, it cannot translate in the z axis as well. I guess it is understood that a rotation along the x or y axes would mean that the robot either summersaults or topples; hence, it is not possible.

Hence, our robot equation is as follows:

Here, x', y', and θ' constitute the robot's final pose,  is the angular velocity of the robot, and  is the time interval. This is known as the forward kinematics equation since you're determining the robot's pose by knowing the robot's dimensions and velocities.

The unknown variables are as follows:

Here, nl and nr are the encoder counts for the left and right wheels, and l is the length of the wheel axis:

We also have the following:

Here, step is the distance covered by the wheel in each encoder tick.