Question 153909
It's easier to show if you have some matrices but I'll make them up here for example.
Let 
{{{A=(matrix(2,2,2,1,0,1))}}} 
{{{B=(matrix(2,2,5,9,3,4))}}}
{{{x=(matrix(2,2,x[1],x[2],x[3],x[4]))}}}
From that, then
{{{3x=(matrix(2,2,3x[1],3x[2],3x[3],3x[4]))}}}
{{{3x+A=(matrix(2,2,3x[1]+2,3x[2]+1,3x[3],3x[4]+1))=(matrix(2,2,5,9,3,4))}}}
Solve each matrix element equation for {{{x[i]}}},
{{{3x[1]+2=5}}}
{{{3x[1]=3}}}
{{{x[1]=1}}}
.
.
.
{{{3x[2]+1=9}}}
{{{3x[2]=8}}}
{{{x[2]=8/3}}}
.
.
.
{{{3x[3]=3}}}
{{{x[3]=1}}}
.
.
.
{{{3x[4]+1=4}}}
{{{3x[4]=3}}}
{{{x[4]=1}}}
Plugging in all the values,
{{{x=(matrix(2,2,1,8/3,1,1))}}}