It could sometimes be proportional. The reasoning here is that if all the items have different prices it won't be, however if the items you are ordering all have the same price they will be!
Isosceles triangle has 2 sides that are the same length. So what you have is three sides that measure: x, x, and (x-8). 70 = x + x + (x-8).
70 = x + x + x - 8
78 = 3x
x = 26.
and 26 - 8 = 18.
So two of your sides are 26 inches and the last side is 18 inches.
