Explanation:
You didn't attach a picture so i'll just explain how to find the number of stomatas/mm^2.
first you find the surface by multiplying the length by the width
0.1*0.1=0.01mm^2
now you count the numer of stomatas in the picture
in0.01mm^2 you have.........................x stomatas
in 1mm you have ...............................y stomatas
y=100x