Using the slope intercept form of y = mx + b
the "b" is the spot where it crosses the y axis....in your equation, it crosses the y axis at -3.
She then measures the distance from the mirror to the base of the tree ... backwards until she sees the top of the tree reflected in the mirror.
The probability is 90/500 =18%
700*18%= 700 *18/100 = 126
126 customers will be selected to participate in the survey on Saturday