15 a2 - 6ab- 8 a2+ 20 - 5ab - 31 + a2- ab

Step-by-step explanation:

Remove the parentheses: 15a²-6ab+8a²+20+5ab-31+a²-ab Combine like terms: 24a²-2ab-11

24a²-2ab-11

Hi

As you can only add or substract item of the same nature, you must re organise  terms.

so :

A = 15 a2 - 6ab- 8 a2+ 20 - 5ab - 31 + a2- ab  

Here  I have  :   " a²"  ;  "ab"  then numbers.

In general, you start with letters with highest exponant.  If two letters have the same exponent, use alphabetic order.

let's  put order  :

A = 15a²-8a²+a² -6ab-5ab -ab+20-31  

Now you  add or substract item of the same kind :  

A = 8a² -12ab - 9

Here I can not do anything else, so calculus is over.