Question

One side of triangle is half the longest side. The third side is

Answer 1

Let S be the first side. 
Longest side is then 2S.
Third side is 2S-10.
Perimeter is sum of all three sides = S+2S+2S-10 = 75
=>
5S-10=75
5S=75+10=85
S=17

Therefore
S=17 is the first side. 
Longest side is then 2S=2*17=34.
Third side is 2S-10=2*17-10=24.