The answer is; 0.88 mm
Because based on estimated, the mountain looks to be smaller than 1 cm in the scale, we will convert the centimeters to millimeter;
10 mm = 1 cm
Therefore 10 mm = 100 km
If 100km are represented by 10mm, how much mm will represent 8.8 km;
(8.8/100) * 10 = 0.88 mm