(b) The image below shows part of the surface of a leaf.
0.1 mm x 0.1 mm
The length and width of this piece of leaf surface are both 0.1 mm

Calculate the number of stomata per mm of this leaf surface.

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 havex stomatas

in 1mm you have y stomatas

y=100x

to figure the total magnification of an image that you are viewing through the microscope is really quite simple. to get the total magnification take the power of the objective (4x, 10x, 40x) and multiply by the power of the eyepiece, (usually 10x)