The equation of a line can be written as:
y = mx + b
Where m is the slope and b is the y-intercept.
For the word problem, the slope is -10 and the intercept is 50, thus the equation is:
y = -10x + 50
If x is the number of weeks, then y decreases by 10 when x increases by 1. We expect that the value of y goes from 50 down to 40, 30, 20, etc.
The example of the friend is that he started with