Hello.

which parts giving you a headache?

Just draw a triangle and list the variables.

So:

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. =)