code: public static function SLN($cost, $salvage, $life)
      {
          $cost = Functions::flattenSingleValue($cost);
          $salvage = Functions::flattenSingleValue($salvage);
          $life = Functions::flattenSingleValue($life);
  
          // Calculate
          if ((is_numeric($cost)) && (is_numeric($salvage)) && (is_numeric($life))) {
              if ($life < 0) {
                  return Functions::NAN();
              }
  
              return ($cost - $salvage) / $life;
          }
  
          return Functions::VALUE();
      }
docs: SLN.
  
  Returns the straight-line depreciation of an asset for one period
  
  @param mixed $cost Initial cost of the asset
  @param mixed $salvage Value at the end of the depreciation
  @param mixed $life Number of periods over which the asset is depreciated
  
  @return float