How do I solve this problem? What is the average acceleration of a car that accelerates from 10m/s forward to 20m/s forward in 5.0s ?

Hint: The above problem is based on the average acceleration of the car.
Average acceleration is the rate of change of velocity, both in change of magnitude and direction.
Average acceleration is given by;
$a = \dfrac{{v - u}}{t}$ or $\dfrac{{dv}}{{dt}}$ (a is the acceleration, v is the final velocity and u is the initial velocity)
Using the above formula we will find the average acceleration.

Complete step by step answer:
Let's discuss acceleration in more details before going through the calculation part.
Acceleration is the rate of change of velocity of an object with respect to time. The orientation of an object is the orientation of the net force acting on that object. The magnitude of the object's acceleration is the combined effect of the net balance of all external forces acting on that object and the object's acceleration is inversely proportional to the mass of the object.
Now, we will perform the calculation part of the problem.
We have initial and final velocity given in the question which are
10m/s and 20m/s respectively.
Therefore, acceleration of the car is;
$ \Rightarrow a = \dfrac{{20 - 10}}{5}$ (Subtracting the two velocities and divided by time)
$ \Rightarrow a = \dfrac{{10}}{5} = 2$
$ \Rightarrow 2m/s$ is the required acceleration of the car.

Note: Acceleration has many different forms such as an object moving in a circular motion- such as a satellite orbiting the Earth- is accelerating due to the change of direction of motion, although its speed may be constant, this is the case of centripetal acceleration. Similarly we have tangential acceleration etc. Acceleration of an object is measured by an accelerometer.