Thanks. I got the awnser like this:

$${\mathtt{x}} = {\mathtt{10}}$$

$${\mathtt{y}} = {\mathtt{3}}$$

$${f}{\left({\mathtt{a}}\right)} = {\mathtt{xa}}{\mathtt{\,\small\textbf+\,}}{y}{\left({\mathtt{10}}{\mathtt{\,-\,}}{\mathtt{a}}\right)}$$

$${f}{\left({\mathtt{6}}\right)} = {\mathtt{48}}$$

$${f}{\left({\mathtt{7}}\right)} = {\mathtt{61}}$$

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