herr

12a+3(a-2)-(a-5)
12a+3(a-2)-(a-5)=12a + 3(1a+(-2)+-1(1a+(-5a)
=12a+3(1a)+3(-2)+-1(1a)+-1(-5a)
=12a+3a+-6+-1a+5a
=(12a+3a+-1a+5a)+-6
=19a+-6

So the final anwser is 19a+(-6)