Answer:
[tex]x=5/2[/tex]
Step-by-step explanation:
When [tex]f(x)=30[/tex], we have [tex]3x+9x=30[/tex].
Combining like terms on the right side, we have [tex]12x=30[/tex].
Finally, dividing by [tex]12[/tex] we get [tex]x=30/12[/tex].
We note that [tex]30=6*5[/tex] and [tex]12=6*2[/tex], so [tex]30/12=5/2[/tex].
So, [tex]\boxed{x=5/2}[/tex] and we're done!
