Slope is rise over run or Y2 - Y1 divided X2 - X1. The X's and Y's are your coordinate points. It doesn't matter which Y you put down first but remember to choose the X in the same place. Your coordinate points are (0, -1) and (-4, 2)
Your equation will be:
-1 - 2 / 0 - (-4)
-3 / 0 + 4
-3 / 4
Your slope is negative 3/4