a)x-12=-28
x=(-28)+12
x=-16
vậy x=-16
b)20+8(x+3)=5^2.4
20+8(x+3)=25.4
20+8(x+3)=100
8(x+3)=100-20
8(x+3)=80
x+3=80:8
x+3=10
x=10-3
x=7
vậy x=7
\(a,x-12=-28\)
\(x=-28+12\)
\(x=-16\)
\(b,20+8\left(x+3\right)=5^2.4\)
\(20+8\left(x+3\right)=100\)
\(8\left(x+3\right)=100-20\)
\(8\left(x+3\right)=80\)
\(x+3=80:8\)
\(x+3=10\)
\(x=10-3\)
\(x=7\)