Answer
A force of $776.5~lb$ keeps the car parked on the hill.
Work Step by Step
Let $F$ be the force directed up the hill that keeps the car from sliding down.
The car's weight of $3000~lb$ is directed straight down. The force $F$ is directed up the hill at an angle of $15^{\circ}$ above the horizontal.
We can draw a right angle triangle with the $3000~lb$ as the hypotenuse of the triangle and the force $F$ opposite the angle of $15^{\circ}$.
We can find the force $F$:
$\frac{F}{3000~lb} = sin~15^{\circ}$
$F = (3000~lb)~sin~15^{\circ}$
$F = 776.5~lb$
A force of $776.5~lb$ keeps the car parked on the hill.
