Plug in your values and rearrange
Actual Size= image size/magnification
But change image size into micrometers
The image and actual size have to have the same units so if the image is 6.9mm that is 6.9x 10^3 um.
10 = 6.9x10^3 / Actual
Actual size = 6.9 x 10^3 / 10
Actual size = 690um