So if they are similar then just try to figure out which two sides correspond to each other from the first and then the second triangle.
See how in the above example I did 20 and 5 not 20 and 14 or 20 and 3.5 because they do not correspond with one another.
Then figure out, is x on the mama or baby shape and either multiply the number received from all proportions or divide.
Multiply : small to big ( # missing on the bigger one )
Divide : big to small ( # missing on the smaller one)
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I hope that helps you out!! Any more questions??