Answer
$29^{\circ}$.
Work Step by Step
From the textbook, “An object whose center of gravity above its base of support will be stable if a vertical line projected downward from the center of gravity falls within the base of support.”
The truck just begins to tip when a vertical line down from the center of gravity is at the left wheel. This is an unstable equilibrium, and the truck falls over if the road is inclined any more.
As seen in the book’s diagram, at that point, the tangent of the angle is 1.2 m divided by 2.2 m.
$$tan \; \theta=\frac{1.2\;m}{2.2\;m}$$
$$\theta = tan^{-1}\frac{1.2\;m}{2.2\;m}=29^{\circ}$$
