Thanks. I got the awnser like this:
$${\mathtt{x}} = {\mathtt{10}}$$
$${\mathtt{y}} = {\mathtt{3}}$$
$${f}{\left({\mathtt{a}}\right)} = {\mathtt{xa}}{\mathtt{\,\small\textbf+\,}}{y}{\left({\mathtt{10}}{\mathtt{\,-\,}}{\mathtt{a}}\right)}$$
$${f}{\left({\mathtt{6}}\right)} = {\mathtt{48}}$$
$${f}{\left({\mathtt{7}}\right)} = {\mathtt{61}}$$
.