Question

Is -7(a+b+c) - [-5(a+b+c)] a trinomial

Answer 1

$-7(a+b+c) - (-5(a+b+c))=\\-7a-7b-7c-(-5a-5b-5c)=\\-7a-7b-7c+5a+5b+5c=\\-2a-2b-2c$

It consists of 3 monomials, so yes, it is.