3.5cm x 4.5cm at 100 dpi is equivalent to 138 x 177 pixels.
The given dimensions are 3.5cm x 4.5cm
Since, 1 inch is equal to 2.54 cm
3.5 cm x 4.5cm is equal to 1.38in x 1.77 in
Now, the given dpi (dots per inch) is 100 that means there are 100 dots or pixels in 1 inch
Hence, the dimensions in pixels will be 1.38*100 x 1.77*100 pixels i.e. 138 x 177 pixels.