Use horizontal method to solve the following problems. (with a sum)

1. find the sum of 3x²-x-12 and -x+21.2. find the sum of 4+3y² and 4+4y-4y²3. subtract -4s³+4st-3t² from 3a²+ab-2b².4. subtract-2a²+4ab -3t²+st-2s³.

please say me in the answer a with a sum please