The basic proportion we have to use is:
[tex]d_o:d_i = h_o:h_i[/tex]
where [tex]d_o[/tex] is the distance of the object from the lens (40 mm), [tex]d_i[/tex] the distance of the image from the lens (120 mm), [tex]h_o[/tex] the height of the object (70 mm) and [tex]h_i[/tex] the height of the image (that we have to find). Solving for [tex]h_i[/tex] we find
[tex]h_i = \frac{d_i \cdot h_o}{d_o} = \frac{(120 mm)(70 mm)}{40 mm}=210 mm [/tex]
so, the height of the image is 210 mm.
