3.x+12+6.x=39
\(\Leftrightarrow9x+12=39\)
\(\Leftrightarrow9x=39-12\)
\(\Leftrightarrow9x=27\)
\(\Rightarrow x=27:9=3\)
3x + 12 + 6x = 39
\(\Rightarrow\) ( 3x + 6x ) + 12 = 39
\(\Rightarrow\) 9x + 12 = 39
9x = 39 - 12
9x = 27
x = 27 : 9
x = 3