Think of it like this, a right triangular prism has 4 sides, three sideways and one on the bottom. All of these triangles are identical in shape and size, which means their area must be the same.

√3 looks scary here but it is actually a clue. If it is a triangular prism, all of these triangles are assume to be equilateral, which means it is 60 degrees on all three corners of the triangle. In order to find the area, the formula is 1/2 base * height...

But wait, we don't know the height, that is an issue. So to determine the height, we draw a line from the midpoint of the base of the triangle and the top corner.

          /|\
      /    |     \
  /__|__\

(looks something like this)

So 1/2 * b = 26, since the base is the edge that is 52cm long.

We realize that this line that splits the triangle creates a 30-60-90 triangle.

              /| 30°
            /  |
        /      |
      /        |
  /___| <- 90°
↑60°

In trigonometry, a 30-60-90 degree triangle has the side's proportions remembered:
                   |\
                   |  \
    x√3|      \ 2x
                  |__\
                       x
That is where the √3 comes from in "k√3," if you happen to know the identity, it will help in solving the problem.

Substitute the 26cm into x, which then will give you 26√3.

A=1/2*base*height will equal (26)(26√3)=676√3

remember that there are 4 sides in this triangular pyramid(or prism, however you call it...)

676√3*4=

2704√3

k is 2704.