更新1:
Pam walks along a road which can be modeled by the equation y=3x, where (0,0) represents her starting point. When she reaches the point (7,21), she turns right, so that she is traveling perpendicular to the original road, until she stops at a point which is due east of her starting point (in other words, on the x-axis). What is the point where Pam stops?
