Equation for trendline in apps script?

I am trying to use apps script to automatically build charts from a google spreadsheet, but also project what may happen in the future using trendlines. I have figured out how to create the chart with a trendline equation in the title, but I need to do a few more steps:

1. Create multiple types of trendlines (linear, exponential, different types of polynomials, logarithmic, etc.)

2. Compare the R^2 values of each trendline and use the one closest to 1 to do step 3.

3. Extract the exact equation of the trendline and use it to project data.

4. Place data in spreadsheet

Here is what I have so far:

    function createChart() {

      var trendlinesopt = {
        0: {

          type: 'linear',
          color: 'black',
          lineWidth: 1,
          opacity: 0.2,
          showR2: true,
          visibleInLegend: true,
    
        }
    
      };

      var chart = formulaSheet.newChart().asScatterChart()
        .addRange(formulaSheet.getRange(2,6,15,2))
        .setPosition(1,1,5,5)
        .setOption("trendlines",trendlinesopt)
        .build();

      formulaSheet.insertChart(chart);

    }

I can generate multiple charts easily, this is just consolidated to the linear function. How do I extract the R^2 value and the equation of the trendline?