You have to isolate the variable. Get the variable on one side of the equals sign, by itself and the number on the other side will be what number the variable is equivalent to. You do this by canceling out the numbers on the side with the variable with reverse operations. Whatever you do to one side of the equals sign, you must do to the other.
EX.
a+5=14-9+2
First solve the right side of the equation.
14-9=5 5+2=7
a+5=7
Now isolate the variable by subtracting 5 from both sides.
a+5-5=a 7-5=2
a=2
So the variable (a) is equivalent to 2.