[tex]2.5x+4x=39\\6.5x=39\\x=39/6.5\\x=6[/tex]
[tex]2.50=2.5\\\\2.5x+4x=39\\\\6.5x=39\qquad\text{divide both sides by 6.5}\\\\x=\dfrac{39}{6.5}\\\\x=\dfrac{390}{65}\\\\\boxed{x=6}[/tex]
