Think of 2 points, (1200,$5050) and (500,$2250), on a straight line. Find the slope and y-intercept of this line:
5050-2050 3000
m = ---------------- = ----------- = 30/7
1200-500 700
Now let's write out the slope-intercept form of the equation of a straight line:
y = mx + b. Substituting 30/7 for m, 500 for x and 2250 for y, we get:
2250 = (30/7)(500) + b. This can be solved for b: b = 107.14 (or 750/7).
Then the linear function is y = (1/7)(x + 750).