\(\Rightarrow3x+13=28\Rightarrow3x=15\Rightarrow x=5\)
(x + 1) + (x + 5) + (x + 7) = 28
<=> x + 1 + x + 5 + x + 7 = 28
<=> x + x + x + 1 + 5 + 7 = 28
<=> 3x + 13 = 28
<=> 3x = 15
<=> x = 5
\(=x+1+x+5+x+7=28< =>3x=28-13< =>3x=15< =>x=5\)
\(\left(x+1\right)+\left(x+5\right)+\left(x+7\right)=28\)
\(\Leftrightarrow3x=15\)
hay x=5