Theory:

Similar to addition, we can subtract algebraic expressions by adding the coefficient of like terms together. i.e. variable coefficient with its similar variable coefficient and constant with constant except that the fact we will include the term additive inverse.
Example:
To subtract  (12a+7) from  (30a2), we have to add the additive inverse of (12a+7) with (30a2).
 
Additive inverse of (12a+7) is (12a+7)\(=\)(12a7)
 
Therefore,
(30a2)(12a+7)=(30a2)+(12a7)=(30a12a)+(27)=18a9
Important!
  1. Subtracting \(2x\) is the same as adding \(-2x\) and subtracting \(-11a\) is same as adding \(11a\).
  2. \(x\) is the additive inverse of \(-x\) and they are equal in numeric value with opposite sign.