Answer:
34 centimetres
Explanation:
Since the flag is made of a square white flags, then all its sides are equal in length. a diagonal of a square would divide it into two equal triangle.
Let the length of the diagonal be represented by t, such that;
=
+

=
+

= 144 + 144
= 288
t =

t = 16.9706
t = 17 centimetres
The length of a diagonal of the flag is 17 centimetres.
So that,
The centimetres of ribbon required to make the "X" = 2 x diagonal
= 2 x 17
= 34
The centimetres of ribbon required to make the "X" is 34 centimetres.