Check the picture below.
so hmm an isosceles triangle has two twin sides, so we know the third side must be either 26 or 54 hmmm which is it?
can we make a triangle with 26, 26 and 54?
we have three sides, let's look at the two smaller sides first.
if we move the sides closer and ever closer to each other, to the extent that one is right on top of the other, what is the length of the red side? Well, assuming the two smaller sides are one pancaked on top of the other, the red side will be as long as 26 - 26 = 0. However, the sides can't be on top of each other, because if that's so, we have a flat-line, and thus we wouldn't have a triangle. So whatever the third side may be, it must be greater than 0.
Now, if we move the sides away from each other, farther and farther to the extent that one is parallel to the other, then the third side will just be as long as 26 + 26 = 52. However, we can't do that, because if that were to happen, we again will have a flat-line and not a triangle. So whatever the third side may be, it must be less than 52.
so the third side for an isosceles triangle with sides of 26 and 26, must be greater than 0 and less than 52, so we can't use 54 for the third side, is too long, it won't fit with our 26 and 26 sides.
thus the third side for an isosceles triangle with sides 26 and 54, must be 54, because we can make a triangle with 26, 54 and 54.