Question

If f(x) = 3x +9 x when f(x) = 30

Answer 1

`x=5/2`

Explanation:

When
`f(x)=30`, we have
`3x+9x=30`.

Combining like terms on the right side, we have
`12x=30`.

Finally, dividing by
`12` we get
`x=30/12`.

We note that
`30=6*5` and
`12=6*2`, so
`30/12=5/2`.

So,
`\boxed{x=5/2}` and we're done!