With algebra there are 2 rules to abide to which will always see you through!
In order to come to the correct answer you need to work backwards to isolate ( i.e do the opposite) the required component this means if there is an addition you should subtract and if there is a multiplication divide. Ensure you do this to both sides and you wont go wrong. Algebra is methodical and if you follow the steps you wont go wrong!
View the problem like a tight knot you need to unravel, take small methodical steps and eventually you will get there. Just one step can make the hardest of equations turn into a basic one.
Read the question the majority of mistakes that are made in exams are because students misread the questions and do not use the information given them. Always use the information in the question to help you.