Answer
$H(x)=\dfrac{3x-x^3}{[your|age]}$
Work Step by Step
Multiplies a number $x$ by $3$ means $3x$.
Cube of $x$ means $x^3$.
Thus, "multiplies a number $x$ by $3$ and then subtracts the cube of $x$" means
$3x-x^3$.
The result divided by your age means $\dfrac{3x-x^3}{\text{"your age"}}$.
Thus, the function is
$H(x)=\dfrac{3x-x^3}{[your|age]}$
