How to calculate new velocity from present velocity, refrence deceleration and time step?

I want to decelerate a car based on deceleration of another refrence car. I have a simulink block which has following inputs: own_car_present_velocity(u), acceleration/deceleration value from another car(a),and I will also need time to implement following formula v = u + (a*t). But here as my solver is variable step solver, how do I always get the value of these variable time steps so that I can correctly calculate the final velocity(v) after every time step?