2.5+13-15-=0.5-b
2.5b+13b-15b-b=-0.5b
18+(-20-8)-n=18-28-n=-n-10=-(n+10)
18n+(-20n-8n)-n=18n-28n-n=-11n
-(m-b)+(m+2b)=-m+b+m+2b=+b
-(a+b)-(2a+3b)=-a-b-2a-3b=-(3a+4b)
n-(m-n)-(-m+4n)=n-m+n+m-4n=-2n
a-b+3b-(5a-7b)=a-b+3b-5a+7b=9b-4a
-15a+3b-12a+20a-7b-2b=-7a-6b=0.7-1.8=-1.1
x7-2x+4=17
-x=6
x=-6
8a+3a=11
11a=11
a=1
3/4b+3 1/4b=-8
4b=-8
b=-2