Hi @Branislav Hopefully you can see that the legend (ie., the right panel above that is labelled "This is the legend ...") is meant to identify the values in the left panel. For example, the first row of values (on the right) includes: (0.650), (1.101), 686.032. Their corresponding lables (on the right) are β(2), β(2), and β(0). So the bottom of the "Excel Linest(.) regression output" panel are the values: ESS = 64,864, SSR = 87,245. Immediately above 87,245 are the degrees of freedom, df = 417. ESS stands for explained sum of squares, and SSR stands for sum of squared residual (aka, residual sum of squares, RSS).
SER = sqrt(SSR/df); in this case, SER = sqrt( 87,245 / 417). The df = 417 because the regression has three coefficients (two slope coefficients) including the intercept. I hope that's helpful!