First you need to understand the problem
if the problem is about area or perimeter you also need to know the formula

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Patience is the key.
And then understand, try to practice yourself first with easy ones, and then with hard ones so that your mind will be practiced.
And then after that, you know the problem, what is being given, asked, then you should know the formula. Algebraic expressions, evaluating theses, linear equations, etc. is very useful on solving math problems.
Next, you can use calculator and then check your answer.