The ratio of the perimeters of two similar triangles is equal to the ratio of their corresponding sides.

Use the ratio of the perimeters to find the ratio of the corresponding sides, then use that ratio to find the corresponding side of the second triangle.

The equation is perimeter of first triangle/perimeter of second triangle=30/20

Substitute the known value for the side of the first triangle 12 to find the corresponding side of the second triangle.

12/x=30/20

Solving above equation we get

The corresponding side of the second triangle is 8 c.m.