Math/geometry help

if I need the diagonal of a square to be 6.5 inches, how long do the sides need to be?

Quoting Kelly&Coralie:" if I need the diagonal of a square to be 6.5 inches, how long do the sides need to be?"

I know for a triangle A^2 +B^2=C^2
C being the longest side, can you do that and just put the 2 triangles together to make a square? Sorry if it doesnt make sense, Im terrible at explaining