If More Variables are Unknown, Solve them First 

This is a common problem in problem solving. For example, you might be asked to find the Torque of a rotating stick but you are only given with the mass that makes it rotate. 
We know that Torque is equal to Force x Length of the lever arm. But we do not know the Force applied by the mass.
To solve this, you have to get the Force using the given mass. Force due to the weight of an object is equal to Mass x Acceleration due to gravity.

Solve the Problem Using the Formula 

Now that you have all the variables except the one unknown, you can solve for the answer.
If you think this is the part where you have the most difficulty, I have only one tip for you:

Practice Makes Perfect!

State the Answer to the Problem

Although you may stop when you arrived at the answer, it is always a good practice to state the answer of the problem.
You may write:
                      Therefore, the acceleration of the car is -10 m/s2.

Now, share this article to that friend who is always bothering you.

(function(d, s, id) {
var js, fjs = d.getElementsByTagName(s)[0];
if (d.getElementById(id)) return;
js = d.createElement(s); js.id = id;
js.src = “//connect.facebook.net/en_US/all.js#xfbml=1&appId=204102246366851”;
fjs.parentNode.insertBefore(js, fjs);
}(document, ‘script’, ‘facebook-jssdk’));