Absolute value

Split this into 3 cases.

$If \;\;x< 2  \qquad  then \qquad  5=2-x+7-x  \quad etc\\ If \;\;2\le x\le 7  \qquad  then  ?\\ If \;\;x>7  \qquad  then  ?\\$

You see if you can work it out now.