i think that the answer is: d=12m
Because first we have to find the variables.
m: is the number of times that you cut on the grass.
AND
d: is the amount of money that you earn.
So, the side of the line is 12 that means the each time that you cut the grass you earn.
So the equation of the amount of dollars which is "d", you like mow the law m times is:
d=12m
i hope its helpful