The lateral magnification m of an optic element is defined as the ratio of  image height to object height:
    m = hi/ho    (definition)
It follows from the Lens Equation that the magnification can be calculated as:
    m = - di/do
The negative sign is needed to conform to the convention that  magnification is negative if the image is inverted, and positive if the image is upright.

The  net magnification of a system of two lenses is equal to the product of the magnification of each lens:
    m12 = m1*m2