In this case, we'll have to carry out several steps to find the solution. 
Step 01:
Data
Triangle 1:
side 1 = 7 
side 2 = 8 
side 3 = 10 
Triangle 2:
Perimeter = 100 
shortest side = ? 
Step 02:
Triangle 1: 
Perimeter = 7 + 8 + 10 = 25
Triangle 2:
shortest side 
We must apply the properties of similar triangles.

25 * shortest side = 100 *7 
shortest side = (100 *7) / 25 = 28
The answer is:
The lenght of the shortest side is 28.