Slope (m) is rise/run. Rise on Y-axis and run on X-axis. Therefore...
m = (y2 - y1) / (x2 - x1)
You have two points: 1: (6, -12) and 2: (15, -3)
So... x1 = 6 and y1 = -12
And... x2 = 15 and y2 = -3
Plug and chug:
m = (-3 - (-12)) / (15 - 6)
m = (-3 + 12) / (9)
m = (9) / (9)
m = 1
Your slope is 1!