Answer:
12.)-5
13.)-16
14)-20
15)6
16)-13
17)1
18)-13
19)15
20)-10
Explanation:
12.)-x=2+3
x=5/-1
so,x=-5
13.)-4=(-2×8+x)/8
 -4×8=-16+x
 -32+16=x
 so,x=-16
14)(7×4+m)/4=2
 28+m=8
so,m=-20
15)4n=34-10
 n=24/4
 so,n=6
16)-2a=22+4
 a=26/-2
 so,a=-13
17)a+3=2×2
 a=4-3
 so,a=1
18)-133+3=10x
 -130=10x
 so,x=-13
19)4+1=v/3
 5*3=v
so,v=15
20)1=3+r/5
 now, 
Take LCM,
We get,
1=(3*5+r*1)/5
now,
by moving 5 into LHS
We get,
1*5=15+r
5=15+r
by moving 15 into left hand side,
We get ,
5-15=r
so,r=-10 because when we subtract smaller from bigger we have to take the sign of bigger.