This is a rhombus is made of 4 special right triangles. 
These are 5-12-13 right triangles.

Each of them has shorter leg = 5, hypotenuse = 13, and therefore
longer leg = 

So each 5-12-13 right triangle below has base = 12 and height = 5

So each of the four 5-12-13 right triangles has area

 
 
and since there are 4 of them, the total area =  cm2.

I think you are studying special right triangles, so this is a good
page to look at:

https://flexbooks.ck12.org/cbook/ck-12-precalculus-concepts-2.0/section/4.3/primary/lesson/special-right-triangles-pcalc/

Edwin