Answer
a) $\sum a=9m/s^2$
b) The direction is toward the center of rotation.
Work Step by Step
a) The magnitude of the car's total acceleration is $\sum a=\sqrt{a_c^2+a_T^2}$
Since the car has a constant tangential speed, $a_T=0$. So $\sum a=a_c$
The car's centripetal acceleration can be calculated by $$a_c=\frac{v_T^2}{r}=\frac{75^2}{625}=9m/s^2$$
b) Since $\sum\vec{a}=\vec{a_c}$, the direction of the total acceleration is the direction of $\vec{a_c}$, which is toward the center of rotation.
