When you translate 4 units to the right, you must add 4 units to the x-coordinate; and when you translate 3 units down you must substract 3 units to the y-coordinate. Then if you translate a point P(x,y) 4 units right and 3 units down, the new coordinates of the point will be P'(x+4,y-3).
Since she has a base amount of $500, that will be your y-intercept. 20 is going to be your slope, and since she's withdrawing it, it will be -20. Here's how the equation would be set up (in y = mx + b):
