Comming from Easylanguage calculating a slope of, ie, a MovingAverage was as simple

MASlope = MA - MA[1];

In C (or lite-C, haven't figured out the difference yet) I tried

vars Prices = series(priceC());
vars MA = series(MovingAverage(Prices,20,MAType_SMA));

vars MASlope = series(MA- MA[1],300)); //300 is the Lookback
or why not MASlope = MA - MA[1]; ?

Appreciate a hint.
Thank you