A car is moving on a plane inclined at 30 ° to the horizontal with...

First, make the free body diagram, of an inclined plane with acceleration in an inclined plane moving in an upward direction.

With respect to the car, the F.B.D. is as follows at equilibrium, $T$ is equal and opposite to the resultant of ${a}_{1}$ ​ and ${a}_{2}$ ​ As ${a}_{1} + {a}_{2}$ ​ makes $30 °$with vertical; $T$ and, the string is also making $30 °$ with vertical.