Answer
The car could decelerate at a rate of $-5.3 ~m/s^2$
Work Step by Step
We can use the rate of deceleration to find the coefficient of static friction $\mu_s$. On a level road, the force of friction provides the force for a car to decelerate.
$F_f = ma$
$mg~\mu_s = ma$
$\mu_s = \frac{a}{g} = \frac{3.80~m/s^2}{g}$
When the car is travelling uphill, the forces which decelerate the car are the friction force as well as the component of the car's weight which is directed down the hill. We can use a force equation to find the magnitude of the deceleration.
$ma = \sum F$
$ma = mg~sin(\theta)+ mg~cos(\theta)\cdot \mu_s$
$a = g~sin(\theta)+ g~cos(\theta)\cdot (3.80~m/s^2)/g$
$a = g~sin(\theta)+ cos(\theta)\cdot (3.80~m/s^2)$
$a = (9.80~m/s^2)~sin(9.3^{\circ})+ cos(9.3^{\circ})\cdot (3.80~m/s^2)$
$a = 5.33~m/s^2$
The car could decelerate at a rate of $-5.3 ~m/s^2$
