The easiest way to do this is to add or subtract the value of the variable.
For example in the equation 5+a=7, subtract a from both sides to get 5=7-a.
If the initial equation was 5-a=7, add from both sides (5=a+7).
Basically, if the initial value is negative, add its value to cancel to zero.
Do the opposite if the initial value is positive.