Linear Functions

The table shows pairs $(x, f(x))$.

<table> <thead> <tr> <th>$x$</th> <th>1</th> <th>3</th> <th>5</th> <th>7</th> </tr> </thead> <tbody><tr> <td>$f(x)$</td> <td>2</td> <td>8</td> <td>14</td> <td>20</td> </tr> <tr> <td>Which formula represents $f$?</td> <td></td> <td></td> <td></td> <td></td> </tr> </tbody></table>