You haven’t given any other information, but here’s how to go about it:
Find the ratio of one pair of corresponding sides of the two triangles. Use this ratio to find the missing side length.
For example if one triangle A has the side lengths 5, 10, and 13 and the other similar triangle B has known side lengths of 10, 20 and one of the side lengths is missing, then the ratio of the corresponding side lengths of B to A is 10:5 or 20:10 which is 2. So corresponding sides in triangle B are 2 times the length of those in A, which means the missing side length in B is 13*2= 26.
I hope this helps you understand how to solve your problem.