Sometimes problems can involve many equations. You need to get from point A to point Z. You are given initial conditions and are asked to find a final solution. You need to figure out which equations you can use that will utilize the initial conditions and also include the final solution. Sometimes a problem will involve many equations and back substitution is necessary. Usually, as the number of equations required increases. You are better off to write down all the equations of interest and the initial conditions and the form of the final value, such as what units is it in, or is it a force, stress, etc. Then by seeing everything in front of you, you can start eliminating equations that do not aid in the solution process.
Make sure you are using correct units
Make sure units are in agreement
Are you using radius or diameter in your calculation?
Sometimes you are given a problem and you don't know where to start. Well, do you know what the problem asks you to find? Maybe you can start from there. Develop equations that have the desired outcome as part of that equation. See if there are given conditions in the problem statement that are also in the equation. If not, think about equations that might relate the given initial conditions to the desired problem outcome.
Some freshman- and sophmore-level subjects such as physics, chemistry, and engineering mechanics involve learning equations and then to "plug and chug" them on calculators. Other subjects, such as thermodynamics and fluid mechanics require much more for proper analysis of a problem. Oftentimes, in these subjects, students must first assess the problem, make and justify assumptions and/or approximations, apply the relevant physical laws in their proper forms, and solve the resulting equations before ever plugging any numbers into a calculator. Many problems in subjects such as thermodynamics and fluid mechanics require more than just knowledge of the subject, but also physical intuition and experience.