Slope 1:7 indicates 1 vertical for each 7 horizontal
Now a simple proportion
47.5/h = 1/7
h is how far you went horizontally.
That's all you need. Actually, it's more than is needed.
You have a point (0,0) where you started.
You have a slope -1/7.
You have another point (h,47.5)
Note: I made the slope negative only to indicate that we are digging. You can flip the coordinate axes over if you like, and have the positive y axis pointing down, but this may be confusing.