58 to x should be the ratio of 6/3 or 1/2 if they are similar.
Find out how much bigger 58 is compared to 6, the matching side on the other polygon.
58/6 = ~9.7
The x side should be half of this, just like it is on the smaller polygon.
9.7/2 = 4.85
4.85 x 3 = 14.55