what is 21.3333333333 in a fraction?

0.333333333 ... = 1/3

So you can think of 21.33333 as:

$${\mathtt{21}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{1}}}{{\mathtt{3}}}}$$

Multiply 21 by 3/3 to allow them to be added:

$${\frac{{\mathtt{63}}}{{\mathtt{3}}}}{\mathtt{\,\small\textbf+\,}}{\frac{{\mathtt{1}}}{{\mathtt{3}}}} = {\frac{{\mathtt{64}}}{{\mathtt{3}}}}$$

$${f}{\left({\mathtt{x}}\right)} = \left(\left({\frac{{\mathtt{100}}}{{\mathtt{3}}}}\right){\mathtt{\,-\,}}{\mathtt{33}}\right){\mathtt{\,\small\textbf+\,}}{\mathtt{x}}$$