When you have a + or - directly outside of parenthesis it means that whatever is inside of the parenthesis needs to be multiplied by positive (+) or negative (-) 1
in this problem there are two places where we need to do that
5x+3-2x-8+(-6x)+(-12)
in yellow positive 1 times -6x is -6x
in green positive 1 times -12 is -12
so we can rewrite this as
5x+3-2x-8-6x-12
now we can combine all of the x terms and all of the number terms
5x-2x-6x = -3x
3-8-12 = -17
-3x-17