which parts giving you a headache?
Just draw a triangle and list the variables.
Let M = the Middle length side side
Shortest side = M - 10
Middle length side = M
Longest side = M - 10 + 25 = M + 15
And we know perimeter is 100m.
That would mean Shortest side + Middle length side + Longest side = 100m
I think you should be able to solve it from there. =)