When we are given the angle theta, we are in a state of equilibrium in the y-direction so we can find the Tension.
The dice obey Newton's Second Law in the x-direction to have the same acceleration as the car.
We plug in the value for T we got above below.
Note: This equation also is valid for 
. This would be a case where the car is decelerating (i.e. braking or going in reverse) and the dice move towards the winshield of the car.